Most film composers never see the set of the film they’re working on, but that was not the case for Joel P. West on “The Glass Castle.”

“He was just basically part of the crew, helping us run cables,” laughed the film’s director Destin Daniel Cretton, who goes back with West over a decade to when they graduated from San Diego State to become friends and collaborators in the local arts scene. “While we were shooting, he made various trips to the set to get a feel for how everything was and he was giving me tracks while we were shooting to be inspired by, so it was a very back-and-forth collaboration with him, which was really special.”

Given West’s extraordinarily nimble fingers, you couldn’t blame Cretton for wanting to put them to use as much as possible, but they work a particular magic when placed on a guitar, able to draw out a world of emotion in the quiver of a string. His work on “The Glass Castle” is nothing less than majestic, aurally creating the palace that Rex Walls (Woody Harrelson) envisions for his family in the hills of Welch, West Virginia, though all evidence points to it being as intangible as the music is. Based on Jeannette Walls’ loving memoir recounting a tough childhood, in which she comes to recognize that in buying into Rex’s dream, she’s been drawn into something of a con, not unlike the small-scale hustles he runs at the local pool hall, the film is illuminated by West’s ability to vacillate between capturing the bewitching grandeur of Rex’s imagination with big, sweeping orchestral swoons and choice piano keys that relay the nerves that jangle within the family when the patriarch can’t live up to his impossible promises, leading Jeannette (Brie Larson) to seek out a life in New York as soon as she can.

Just as Jeannette eventually is able to bring all the disparate aspects of her life together to find inner peace, West employs an eclectic array of instruments to detail her inner struggle and give her harmony, just as he did before for Cretton and Larson in their previous collaboration “Short Term 12.” It also all seems to be coming together For the singer/songwriter, who has branched out to work with other filmmakers such as Paul Weitz (“Grandma”) and the Nee Brothers (Band of Robbers”) while continuing to put out albums of his own, recently teaming with Darla Hawn to put out the album “Vaquita” under the aegis of Flood Coats. In the midst of a busy summer, West spoke about his work on “The Glass Castle” and his longtime partnership with Cretton, as well as drawing on the Walls’ archives for inspiration and being able to work with a full orchestra for the first time.

Brie Larson in "The Glass Castle"How did you and Destin first come to collaborate?

Destin and I go back pretty far. We went to the same school in San Diego at different times, and living in San Diego, which is a pretty small creative community, you tend to find the other artists pretty quick. We were both fans of each other’s work and somewhere around 2008, we became friends and he was making some music videos for my band The Tree Ring and I started doing some music for shorter videos he was doing. I had never really considered scoring for picture before, but we’ve just always been a part of each other’s projects.

The first film that we worked on, which was called “I Am Not a Hipster” was something that Destin just wanted to make because he was trying to get another feature made. He just decided we should make a feature that takes advantage of all the creative friends we have, so it was more the idea of me writing some songs to capture the San Diego creative community that we had going. And then “Short Term 12” was the first time I actually did score music for an feature film, so that was a totally new experience for both of us. Since then, I think I’ve made 10 movies, getting way better at what I’m doing and understanding how to support the storytelling, and Destin’s been honing his craft [as well], just being more surrounded by great people up here in L.A. and working on some other side projects, so this was cool because I feel like we learned how to do it together. Even when it’s not direct [colllaboration], we’re always aware of what each other’s working on, giving each other notes, so it’s a long relationship and it’s always cool to get to work together.

Was composing for film a natural extension of your work or was it an adjustment?

I never really watched movies much growing up. It’s just something that wasn’t really on my radar and it ended up being fairly natural because the music that I like to make happens to work for film. I’ve always been a fan of recorded music more than live music and I love to be able to take music on a trip somewhere and have it create a context for other experiences. That’s always how I’ve written songs. When I first started writing songs, it always came from being on a road trip or driving around in a certain setting and trying to find the right song for it, so then I’d want to write something that would work for that moment. That’s the way scores work anyway, where you’re trying to create music that captures a feeling or gives context to another experience, and I’ve just had to do some catching up on the mechanics of how it all works and get more familiar with film.

Ella Anderson and Woody Harrelson in "The Glass Castle"When you and Destin started talking about “The Glass Castle,” what were the early conversations about?

The first thing we did was I just went through [Jeannette Walls’] book and looked for any musical references and if anything, it was ‘60s and ‘70s country stuff that was just on around West Virginia and her dad was a big Johnny Cash, Willie Nelson fan. There’s some references to Kitty Wells and Roy Acuff, so we knew there was something slightly Southern that we should be referencing, but the way Destin usually approaches music, we like to think about what the music should feel like overall before even thinking about scenes.

The main reference that Destin had even before pre-production was actually Christmas music. It was a funny and interesting challenge because he had this idea [of] “I don’t want to have actual Christmas music, but there’s something about the way that Christmas music makes me feel that just feels right for the movie” [in how it’s] nostalgic and there’s kind of a feeling you get looking back at childhood memories. There’s some magic and some pain there, so that’s where we started. I ended up listening to a ton of old orchestral Christmas music and tried to figure out what about it gives you that feeling just besides melodies and lyrics that are Christmasy.

While they were shooting, I created a bunch of music that wasn’t necessarily for any scenes or any specific moments in the movie, but were just little tests to see what felt right to Destin and we narrowed it down to a couple different demos where he felt like these feel the most like the movie. Some of those pieces didn’t even end up going anywhere in the film because they didn’t make sense, but they were wells to draw from [where] I would pull elements from some of those demos to create stuff for scenes that would be more specific to what the scene needed to do. Also, we’re covering a long span between the early ‘70s and mid ‘90s, so we were trying to create something where the instrumentation felt like it worked for all of those eras and wasn’t stuck in one time period.

Woody Harrelson and Naomi Watts in "The Glass Castle"Something else that’s interesting over the course of the film is how the score starts‎ with this grandeur out in the vastness of the desert, but ultimately strips things down as it goes on and Jeannette’s eyes open more to her situation. How did that come into play?

It’s an interesting movie because the change that happens over the two hours is just in Jeannette’s head. The context of her relationship to the family doesn’t really change that much. It’s more about the way she sees it, so trying to track the way she felt when she was four to the way she felt when she was 14 and then 24, the idea was to keep some really basic melodies, so that there was a throughline to come back to and [we could go] full-on trying make those melodies gigantic and [give it an] epic adventure feeling upfront and start whittling them down. The first couple cues in the movie are super overblown, referencing Americana, Aaron Copeland-type music to capture that big childhood feeling, even though you can kind of tell as an audience member, [what’s happening onscreen between Jeannette and her parents is] probably a little fake because you can see what’s going on is a little sketchy. The trick was trying to come up with melodies that were simple enough that you could recognize them even if they were being played by a giant orchestra with horns and big strings going or just being played on a piano really simply, so that we could put whatever clothes on them to fit the way that Jeannette felt in that era.

Was this your first time working with a full orchestra?

It was and if I never get to do it again, I’ll be glad that I took this course just to get to do it the one time because it is a really amazing experience. It was pretty stressful too, just because it’s all prep and then the score is recorded over the period of basically a day and a half and you just hope you get what you wanted. I guess I get to feel what Destin feels like in that regard, just hoping that all the prep works out how you envisioned. But I had a lot of great people around me making sure that it was going to go the way that it should and Lionsgate helped to put together just an awesome team, so I felt very supported. I really hope I get to do it again because it was really magical to hear that many people play music, even simple music.

Brie Larson in "The Glass Castle"You were actually in Welch for the start of shooting – did being there help inform what you wanted to do or what types of instruments you wanted to use for the score?

So much of the story takes place there and the way [Jeannette] talks about that town that I felt like I should go and I went to Welch thinking I would get some musical ideas and I would listen to West Virginia music while I was there, just to try and get my head in it. But what I ended up coming out with was a better understanding of the day-to-day life of those people living there, especially in the post-coal boom. You can tell there was a financially stable time there and now they’re beyond that, so I [had] sympathy for Jeannette’s experience growing up there and her dad’s experience. When you think of Appalachian music in West Virginia, you think about banjos and fiddles and old hymns and Jeannette says it just really wasn’t part of their language. Most people in Welch listen to whatever’s on the radio, so I think I had a romanticized idea of oh, it’s Appalachia, so it’s going to be very bluegrassy and then I got there and it’s not. These are just hard-working people trying to get by and music plays a different role in their life, so [being there] ended up informing the music in a way of having a sensitivity to those real-life experiences rather than actual instrumentation. ‎But the people were so welcoming and sweet, I hope I get to go back.

It’s a beautiful sequence in the film when the Wells arrive in Welch and start turning the ramshackle house into a home, and you build up the score from the thump of a country guitar into a ‎full-force song called “Summer Storm,” which I’ve heard was inspired by a poem Rex had hidden away. What went into that?

It’s one of the benefits of working with someone you know really well, like Destin, [where] we have a good creative shorthand because we’ve worked on so much stuff before. He wanted to have a song that would have a vocal for that scene and we realized pretty quickly that we were going to have an easier time just writing an original song as opposed to go try and find this perfect song to fit there because we figured it was probably going to be a wild goose chase.

[The scene is] all about this family having this excitement to work to put this house together and [Destin] was like, “I would love this to be something that has an old, fun union work song, like a ‘Swing Low Sweet Chariot,'” which is actually one of the songs that Rex Walls loved. He thought it’d be fun [to have] a good beat that feels like a fun sing-along that anybody could hop in on and just start singing, so I was just trying to write something that was simple, like it could be played in that house, just one person with a guitar and people clapping and singing and stomping. Then it was Destin’s idea to try and tap into the poem that Jeannette references in her book, but she didn’t know what the poem was.‎ The song that’s in the movie was actually the second [version]. I wrote a whole other song called “Summer Storm” first that was a little too sentimental and that ended up leading us to something that had a beat that felt more fun and didn’t have as much of a sweetness to it.

Woody Harrelson and Naomi Watts in "The Glass Castle"In general, the tone of this must’ve‎ so difficult to get right, and I know it was a lengthy edit for that reason. Were you working on it throughout that entire process or was the picture more or less set when started getting your hands dirty?

For this one, I was in for the long haul. We used very little temp music editing. I had a lot of score music on the table to start working with on day one of the edit just because I find editors need to start working with music right away‎ and I’d worked on so much music during the shoot, so I like to get some general musical ideas on the table before the edit starts, so that we can zoom in on it. I actually worked out of the editing suite for the whole editing process, which is the longest I’ve worked on something for a film. I made a lot of music that didn’t end up getting used that I made on the fly, just to try scenes or to put into test screenings because this film is very touchy.

We realized pretty early on in test screenings that the wrong temp music could throw off the scene and we would have a really hard time understanding from audience feedback whether or not [it was] the music was pushing them to feel a certain way, so I ended up scoring every single test screening version just so that we had something as presentable as close too the emotional tone we would be using in the final [version] as possible. It ended up being a ton of work and a ton of music thrown out, but I always think it’s totally worth it because by the time we went to score and record the orchestra in Nashville, we were so confident because I had tried so many different things for each scene and pushing every score piece one way or another just to see what would happen and then letting it settle in. Ultimately, it was a long process, but I always think that makes the music more special and letting the edit and the music grow together just ends up creating something that’s a little more cohesive.

What was it like seeing the final product when it all came together?

It’s one of those things I feel so close to, so watching the final cut, especially with color, both feels like an old photo album or something very personal and at the same time, it almost feels like something I can’t believe my friends made because I just love the movie so much. I just have a lot of pride, and for me, the biggest thing was when Jeannette came out to L.A. to watch the movie for the first time. We were trying to take care of her very personal and very complicated story and trying to make it into a movie that’s dramatic, but doesn’t exploit any of the very raw and difficult situations that she went through growing up, and I just felt very grateful getting to see the final product and knowing that Jeannette felt like Destin had taken really good care of her story, and very glad that we put in the time that we did. I really love it.

“The Glass Castle” opens on August 11th.