Before “Marrowbone” premiered recently at the Toronto Film Festival, Sergio G. Sanchez had never shown the film to an audience of more than five or six friends and family members. Naturally, these invites included the film’s cast, except for one — the eight-year-old Matthew Stagg, for whom the chiller might be too intense. Having the experience of writing the J.A. Bayona-directed “The Orphanage” and “The Impossible,” Sanchez has put himself in this unfortunate predicament before, giving young actors plum, demanding roles brimming with intelligence rare for their age in films they probably shouldn’t see until they hit their teens. After being cautious with the script, which Stagg never saw in its entirety, Sanchez decided the best course of action would be to invite the boy’s parents and grandparents – who perhaps in turn would hire a babysitter.

“And they were like, ‘Oh, Matthew would love to come,'” smiles Sanchez, with mild chagrin. “And we’re like, “Really? Because [even during shooting] he doesn’t know [the whole story],” and they were like, ‘Yeah, yeah. I think [he does] … ‘ So [Matthew] sat by my side and I told him, ‘Okay, when I say ‘one,’ you cover your eyes, and when I say ‘two,’ you cover your ears.'”

Stagg survived the screening in fine form, which was more than you might be able to say for some adults who’ve seen “Marrowbone,” a deviously clever potboiler centered on the Fairbarron family, made up of Jack (George MacKay), Billy (Charlie Heaton), Jane (Mia Goth) and Sam (Stagg) and their mother, who has relocated them from England to America before knowing she’ll succumb to poor health. It’s obvious the children have experienced heartache well before her death, but that’s a mystery Sanchez allows to permeate the air as the kids attend to the more practical concern of being split up by child services, setting up house deep in the countryside, forging their mom’s signature when needed and communicating only with a young woman named Allie (Anya Taylor-Joy), who works at the local library. Yet as careful as the quartet are about covering their tracks, a nosy estate lawyer (Kyle Soller) poses a threat on the outside while the creaky house that brings him around has some surprises of its own.

One of the few things that won’t come as a shock about “Marrowbone” is that it is such an accomplished and nerve-jangling thriller even though it is Sanchez’s first time behind the camera, a more-than-worthy entry into the grand tradition of sumptuous gothic horror films that his home country of Spain is known for producing. While in Toronto, Sanchez and his leading man MacKay, who as Jack becomes the de facto patriarch of the clan, spoke about filming in the director’s hometown of Asturias, giving the actors — and the sound designer — a lot to work with, and how a terrifying summer on-screen was actually idyllic off-screen.

Mia Goth, Charlie Heaton and Matthew Stagg in "Marrowbone"How did this film come about?

Sergio G. Sanchez: It actually started with my producer Belén Atienza. We like to play this game whenever we have some free time that we call the pitching game, and [she] was like, “Okay, throw me an idea for something. Don’t even think about it.” So I gave her the seed of this story, and she was really excited about it. She told me, “Why don’t you try and just write three pages a day?” I had no idea where it was going – I only had the concept – but I started sending her three pages every day, and suddenly by the time I got to page 60 I was so in love with the story, I wrote the last 40 pages in one day.

George MacKay: Just the title really, really intrigued me — the word “Marrowbone,” when I went to read for the role of Jack [during the] initial casting, and when I met with Sergio, he gave me context for the story, and the true nature of the whole thing.

Sergio G. Sanchez: Of course, we rewrote it many, many times [after], but [the idea] just came out of nowhere and it has so many of the themes that really interest me, that it just came out naturally. I always say [about] writing that it’s like all of us have hidden treasures and life gives you the tools to dig them out, and this one was just there waiting. Sometimes I would be writing and it felt like that somebody else was doing the writing for me. It felt like being possessed.

How do you create the atmosphere for this film, because as a family, you feel so close onscreen?

George MacKay: Firstly, we just all really got on naturally, and that was a result of those first two weeks. We all met each other, and the rehearsals that we had, we mainly improvised scenarios to give us a past, and that gave us a real sense of who we were in the story. But the process of doing that straightaway when you’re first meeting people is quite exposing in a lovely way. It was a really supportive atmosphere, and we just made sure that we got every meal together and all of that. So it was a mixture of the creative process and this beautiful place that we were in as well [where] it was the cast and crew in a wonderfully fluid, but concentrated environment.

Was this a location you knew well, Sergio?

Sergio G. Sanchez: Yes. It’s all in Asturias, which is in the northwest coast of Spain where I’m from. There were actually some different incarnations of the screenplay [where] it was going to happen in [a different part of] Spain, then it was going to happen in Ireland, and it was in the Ireland stage that I went to visit a castle in Asturias because the kids like to go to a place there – Red Witch Rock, which was meant to be a castle [I knew], and when I was visiting that castle, I saw the rock, and I was like, “Wow, that rock looks amazing,” so I thought, “Forget the castle. It’s a rock, and then I can make it to the American version [of this film].” That’s how I found it.

I was [also] really obsessed with shooting in a real house. I didn’t want a soundstage with green screen outside the windows. Most of the film is shot with available natural light, and it helped having those three interconnected rooms [the siblings] all share. Charlie [Sexton] had this idea that he wanted to spend a night there with all of them, but then we weren’t allowed to, but it would’ve been fun.

George MacKay: [The house] had such a like happy, wonderful, vibrant presence to it in the daytime in that sunshine, but then at night, it was really, there was something eerie about it. After a shooting day, we rehearsed in the evening for a scene that we were doing the next morning and we were there until dark, so when everyone was going back to the base camp, just a couple hundred meters away, they were getting back in the car to go home, and I realized I’d left my script in the bedroom, so I ran back to get it, but it was night and when I ran through the house, and suddenly thought, “Gosh, this is what would it have been like for Jack.” So I thought, “Well, I should get into his bed for a second.” Immediately, I was really quite scared, and I thought, “Right, I’ll count to 10 and I’ll stay there.” I got to 10 and then grabbed my script and bolted out. [laughs]

Mia Goth, Charlie Heaton and Matthew Stagg and George Mackay in "Marrowbone"One of the things I’ve loved about all your films, dating back to your collaborations with J.A. Bayona, is how there are all these tangible artifacts around which speak to so much history of the characters, but must also be useful to the actors playing them. Are those kind of things sprung on them to inspire emotion?

Sergio G. Sanchez: I wanted the house to feel very lived in, but I didn’t want your classic horror story house. I spoke to my production designer and my cinematographer and I told them, “Well, it’s, this is the place where they want to live forever, so it needs to feel like a place where you would want to.”

We gave each of the rooms [a distinctive look], just like Jack had his notebook, and we were careful to put some ink on George’s fingers so that you would always know [he’d been sketching]. Billy does these little carvings, and Jane has her drawings, and Sam has toys – I’m obsessed with those little things. Building the fortress was fun. It was funny because in a way you need to believe that those things were made by someone, so I actually got my sister to do all of the props, and now I have like a little museum of all things “Marrowbone” [at home]. ‎There’s boxes and boxes – I should decorate my room with everything that these guys made.

At the beginning, George was like, “I think I should write [my own notebook],” and I’m like, “No, my sister is doing this.” And then it’s like halfway through it’s like, “Yeah, go ahead and do it.” [laughs]

George MacKay: I really thought I should do it and I’d been practicing drawing. Then I turned up with this stuff that looked like a six year-old. [laughs] And then I was like, “I think maybe, yeah, it’s best if we just let [Sergio’s sister] do it.”

Sound is also such an effective element of the film. Are you actually thinking pretty specifically about the score and the sound design before arriving on set?

Sergio G. Sanchez: Actually, my sound designer always says it’s a pleasure for him to read my screenplays, not because he likes the stories or anything, but because there are sound indications, so there is always stuff to work with. [laughs] Things happen in the movie that we cannot say, but there are hints that have to do with the sound, and a big revelation is actually told with sound design rather than anything else.

With the score, one of the happiest moments in making films is always the moment when you record it with the orchestra, probably because you’re already at the stage where it’s like, “Okay, the film is done. You don’t need to be nervous about anything, it’s just all about just enjoying it.” Working with Fernando [Velazquez] is always a pleasure, and frankly, I think this is his best score. The main theme is so incredible, and it’s nice that it’s a narrative element – each of the characters has an instrument [associated with them], each having like a short [musical] phrase and when you put them all together it’s the theme. I had lots of fun with that.

George Mackay and Anya Taylor-Joy in "Marrowbone"Was there a particularly crazy day of filming on this?

Sergio G. Sanchez: I felt really bad – and really good – the day when George comes running from outside the house all the way up into the hidden place. We’ve broken that shot in three moments [in the edit of the film] because it intertwined with something else, but I wanted to shoot it in the one scene just in case. And [George] did it like 12 times?

George MacKay: Yeah, something like that. So did everyone else, so it was a joint effort.

Sergio G. Sanchez: Yeah, with the camera and the sound people and everyone running. That was hard. I kept saying like, “More. More. More,” and at some point [George] was like, “Am I not giving it to you? You’re not happy.” And I was like, “No, no, Since we have all afternoon to do this, I just want to play and see what we’ll get.” That was a tough day. Then there was another day where they were having a ball, and it was a tough day for me because we only had three hours to get everything that was happening on the beach. [This family-like group of actors were] basically playing for real, and I’m like, “I don’t have time.” [George] was like, “This was my best shooting day ever.” I’m like, “Not mine.” [laughs]

George MacKay: Filming this was really like an emotional process. We really had an amazing summer, and [watching it now], it’s just a wash of all of those feelings. It’s something I’m just really proud to be involved in, because it feels so rich and it’s a film you can watch on so many levels. It’s overwhelming and wonderful.

Sergio G. Sanchez: It was just so incredible to make this movie and work with the wonderful actors and the whole crew. Like George said, I always told my production designer and the cinematographer, “We have to make this feel like it’s the summer you would always want to go back to.” And my memory of the movie feels like that. It’s like I wouldn’t mind doing that again forever.

“Marrowbone” does not yet have U.S. distribution. It will next play at the San Sebastian Film Festival on September 26th, 27th and 28th.