For all the skills Kelly O’Sullivan had accrued as a celebrated stage actress in Chicago, it was one of the jobs she held off-screen that paid great dividends for her work in “Saint Frances,” reliving her experience as a nanny to tend to her six-year-old co-star Ramona Edith Williams.

“We really were friends and hanging out and playing, so eventually she and I developed this ritual of what we would do right before they called ‘Action,’ where she would be on a staircase and I would say, ‘Go,’” O’Sullivan recalls. “She would jump from the staircase into my arms and then I would put her on the mark, and I [thought], ‘This is totally a nanny move to invent a game that results in a child being in a space where you need them to be, to be hitting their mark.’”

It was hardly the only time in either the production of “Saint Frances” or the finished film that O’Sullivan would take something challenging and find the fun in it, considering the implications of motherhood in ways you rarely see onscreen. While every moment in the story of Bridget (O’Sullivan), a 34-year-old au pair who picks up a new job watching over Frances (the effervescent Williams), is undeniably authentic, having grown out of O’Sullivan’s real-life experience in some way, it is also the kind of breezy entertainment you know couldn’t have been easy for the actress and writer and her partner, director Alex Thompson to pull off with uncommon sophistication, shrewdly drawing a parallel between Bridget’s decision to abort an unwanted pregnancy by way of Jace (Max Lipchitz), a guy she’s unsure of committing to, and the couple at the heart of her professional life, Maya (Charin Alvarez) and Annie (Lily Mojekwu), whose love for one another clearly runs deep, yet struggle with the roles they’ve had less and less say in defining for themselves since becoming parents.

Just one look at the precocious Frances and you know that procreation is a beautiful thing, but so too can be finding contentment away from traditional societal benchmarks as “Saint Frances” argues, illustrating the pressure women face in making decisions around their biological clock and having ambitions outside of starting a family with grace and good humor and upending conventional stories around pregnancy to find its own way forward, much like its characters. Filmed with as much light and sensitivity as is inherent to O’Sullivan’s deft screenplay, “Saint Frances” unfolds feeling much like the warm, easygoing summer in Chicago it’s set in and just shy of a year after picking up the Audience Award and a Special Jury Prize upon its premiere at SXSW, followed by many more accolades to come on the festival circuit, O’Sullivan and Thompson graciously took the time to talk about how the film’s naturalism and narrative complexity went hand in hand as the film begins its theatrical run this week, as well as the opportunities opened up by having a young actor as gifted as Williams and the last-minute reshoot that made everything else fall into place.

How did this come about?

Kelly O’Sullivan: I started just writing [from] two points of inspiration in my life — I had been a nanny and that I did get an abortion. I thought the juxtaposition of those two things, if handled with a lot of humor and a light touch, but hopefully nuance and authenticity, could make for an interesting, compelling story and one we haven’t necessarily seen before.

Through the four female characters you have in the film, there really is an entire array of female experience. Did it naturally work out that way or did you design the story with that in mind?

Kelly O’Sullivan: I’m at this very interesting point in my life where all of my friends and I are all in our mid-thirties, so we’re all experiencing different things like motherhood or fertility or choosing not to be mothers, so I feel like I’m in this crucible of experience right now where my friends are just running the gamut, and I knew that I wanted at least a story of motherhood and postpartum and then a story of abortion and choosing not to be a mother at that time. There were other storylines having to do with other experience, like miscarriage and things like that that we cut because we wanted to streamline more, but it was always the intention to have a diverse depiction of women and motherhood.

You’ve said that you had a lot of the actors you had in mind for this, but obviously Ramona is a discovery. Does she change things?

Alex Thompson: I think if anything because of how well she handled the language on the page and how prepared she was coming in every day, she opened up the possibilities of what we were able to do. I don’t think we ever changed a line for her, and in fact, there were whole monologues that we fully anticipated having to cut that she would just suddenly deliver on, so one thing that felt really generous about Ramona’s involvement was just that she’s so real on screen and so authentic, the scenes were able to be more dynamic because we weren’t trying to have to cut around a six-year-old. We were shooting wides and long shots and two shots and we had scenes that would go on for seven or eight minutes and most of the time, she held her own every step of the way.

At first, [our cinematographer] Nate and I had considered a more stately visual style — we were thinking of dollies and these controlled movements, but as soon as we met Ramona, we realized we wanted to have the option of living in the scene a little more actively, so we committed to handheld 40mm lens as our base. [With] the 40, you’re not searching for people, you’re a little bit further back, but not so far back that it looks like an outright comedy, so that became our baseline and then any time we deviated from that and changed that visual language, it was meant for a reason. And all of that came from Ramona and just realizing that she was this vibrant person that it would be silly to try and box her in on a mark.

Is there anything you may not have anticipated, but it’s in the film now and you really like about it?

Kelly O’Sullivan: One of my favorite moments of the whole film is when Charin Alvarez as Maya says that prayer in Spanish over Franny’s hands after she scrapes her hand. That wasn’t scripted, that was in the moment and to me that sums up the whole movie, that this is about healing and somebody helping you heal and assuring you things are going to be okay. That coupled with [Jace’s] roommate who is screaming at the video game — all of that was improvised. In the script it just said, “He screams at the TV,” but the things that he says in it, like he says, “I need healing, I need healing,” as he says “fuck, fuck, I need healing,” that’s a crystallization of this film that could not have been planned.

From what I understand, when you cast Max Lipchitz as Jace, you also got his real-life roommate Danny and his real apartment in the deal. Did that all come as a package?

Alex Thompson: Max and Danny do have a great apartment or did. [laughs] I knew Max was the best person for the part, so that was the first ambition, but my buzz word for the whole thing was “process.” I was like we can’t just barrel ahead. We’ve got to have process, so we saw people at the casting agency for that role, but I surreptitiously did a self-tape with Max and sent it to everyone and they were like, “This guy is great. Wow,” and from there, we cast his roommate Danny, who was also the best person for the part. We’d actually auditioned [Danny] for Jace, and [because] we were tech scouting and stopped by and I was like, “See you at the audition on Monday,” their apartment we got before either of them were cast. [laughs] So it was just perfect and there’s a sense when you walk into a space that’s actually been lived in, a sense of naturalism that you can’t really fake the way they’ve used the space. That was Max’s actual bedroom, not just his apartment.

You used the apartment you actually shared as well for Bridget’s apartment. Was it interesting to look at that space as a film set?

Alex Thompson: We quickly moved out of that apartment. [laughs] We saw it for what it was, which was a pile of trash. Actually, our DP was very against shooting there…

Kelly O’Sullivan: Because it was too small.

Alex Thompson: Too small and it didn’t have good light for him, for his purposes. He actually got very angry at me one night because he put up a light that was shining right in our neighbors’ window and I told him to take it down and he said,” I told you this place was horrible! I told you it wouldn’t work! I told you!” [laughs] We kept up a lot of the art department dressing because it looked better than the way we put it. And it was easy [to shoot there]. We knew where all of the good coffee spots were within walking distance.

I’m speaking out of my depth, but I’ve heard part of the reason this film has such vibrant color is because Nate somehow made it so dark on set that it would have the opposite effect once you brought out the color in post-production. How did that work?

Alex Thompson: Yes, he underexposed the Alexa so extraordinarily, he would ask for 15 minutes to light scenes and we would come back in and couldn’t read the sides in front of us. [laughs] He just took the light away and then we worked with [the colorists at] The Mill on color and they knew exactly what to do with it. Of course, Kelly’s one request visually was that it be a colorful film and that it should be bright and every day we’d come to set and I would have to say “I cannot see Kelly’s face.” But Nate was like, “Trust me. Trust me.” He’s the new Prince of Darkness, I would say.

It looked fantastic. I understand you built pickups into the schedule, which is a real luxury for an indie shoot, and it actually led to that great opening scene in the film where you see Bridget engaging in small talk at a party that gets too big for the guy approaching her.

Kelly O’Sullivan: Yeah, we had a couple of rough cuts of the film and we were showing it to people without that first scene and we realized we needed a scene that set up both expositionally what the audience needed to know because they had no context for who the character was, but also just the tone that hopefully this film is going to have a sense of humor and to start off with some laughs. Initially, the script just started with that sex scene [with Jace] and [the scene we added] ended up being really special. I think that pickup was the last thing that we shot.

Alex Thompson: Yeah, we had the “wrap party,” with very broad quotes because barely anyone was there on the back porch of that apartment for that night.

Kelly O’Sullivan: Yeah, we just had a couple of friends come over to that apartment to try to keep up the look of a party…

Alex Thompson: Yeah, there was six extras in that scene.

Kelly O’Sullivan: They’re also the extras at the baptism. Don’t look too hard, but these are the same group of friends who were on call whenever we needed bodies. [laughs] And Brad, the guy who gives the opening monologue, is also the manager at the restaurant because we just…

Alex Thompson: You can’t get enough Brad.

Kelly O’Sullivan: [laughs] But it was really fun. There was a looseness to saying, “We want to start this movie differently” and then having the ability to do it, so it was nice to shoot a really fun party scene as our last hurrah as a team.

What’s it been like traveling around with this and seeing the reaction to it?

Alex Thompson: It’s been overwhelmingly positive. I don’t know if when we wrapped we had a clear idea of what we made exactly, where it fit the culture as a whole. But seeing the warmth and the generosity from a wildly diverse group of people has been really positive.

Kelly O’Sullivan: It’s been a total surprise and it makes me feel very hopeful in general because I wasn’t sure that people would connect with the story or that it would be really divisive. At least in the conversations we’ve been having, it hasn’t been and we’ll see once this movie opens how it goes, but it’s been really, really affirming and lovely.

“Saint Frances” opens on February 28th in New York at the Angelika Film Center and March 6th in Los Angeles at the Arclight Hollywood and the Laemmle Playhouse 7. A full schedule of cities and dates is here.