Completing a marathon is a huge victory for any runner—but Brittany Runs a Marathon is a victory for the cast, filmmakers, and real-life woman on which it is based.

Previously, the movie's star, Jillian Bell, stood out in 22 Jump Street, joking about how Jonah Hill looked way too old to be in college. You've seen her ensemble work on Workaholics, and she's played supporting roles on The Night Before, Office Christmas Party, Rough Night, and Ryan Hansen Solves Crimes on Television. Now Brittany Runs a Marathon finally gives her the lead.

"It's slightly different than what I pictured my first lead role would be," Bell told Popdust. "This is a movie that feels very personal to me. There are a lot of dramatic elements, and it was so much fun creating this whole character. It's really based on a real person named Brittany O'Neill, who the writer/director Paul Downs Colaizzo wrote this movie about. It was sort of a love letter to his best friend since college. It's a beautiful story."

Colaizzo, who's mostly worked in television development and is making his directorial debut with Brittany Runs a Marathon, told Popdust that he chose to write a story inspired by his friend, because "I wanted to create art that I felt connected to, that I cared about." He was a fan of Bell's work in 22 Jump Street. Now, he's impressed with the range she brought to his film.

"At first, just knowing her work, I didn't know if she'd be able to play a role with this many layers and this much drama, this much vulnerability and humanity," Colaizzo said. "Immediately when we met, she had a protective nature over the character. I was looking at her realizing this is a woman who is perceived as comic relief by the roles and by her films and was looking to challenge herself to change how she sees herself and how the world sees her. So it felt like what Jillian was going through as a performer was exactly what the character would be going through, as well."

This poster shows Brittany before she literally turns her life around. Amazon Studios

"Camera Brittany," as Colaizzo distinguishes the character Bell plays from the real O'Neill, parties on weeknights and doesn't worry about being late for work or bouncing from job to job. She's popular because everybody has a good time around her, but her humor is a defense mechanism. After trying to score some pills, her doctor cautions her about her unhealthy lifestyle. Some neighbors invite Brittany to join their running group, and she reluctantly accepts. Gradually, the discipline helps her get the rest of her life on track.

"It's different from what I'm used to," Bell said. "I think there are definitely moments where she is genuinely just having fun and trying to make people laugh, and then there are moments where she is using it as a crutch."Delving into the psychology of Brittany's humor was part of Colaizzo's mission. In most Hollywood movies, Brittany would be a funny sidekick who'd leave the scene after her punchlines, so the film could re-focus on its star. But Brittany Runs a Marathon takes a different approach.

"The point of the movie was to take stock characters and let them evolve, let them become more dimensionalized," Colaizzo said. "Even the supporting characters become leads in their own story. So we approached it like a drama, so that we're never going for any broad laughter. I think a lot of our humor comes from the relatability of feeling humiliated and exposed." The same treatment is given to funny house-sitter Jern (Utkarsh Ambudkar), who becomes a leading man.

Brittany (Jillian Bell) trains for the marathon Amazon Studios

But even at the beginning of her journey, when she is a mess, Colaizzo is celebrating her. "The film is inspired by her incredible and admirable, inspirational, really galvanizing journey," Colaizzo said. Although the character does differ from her real-life counterpart. "I had to create a different character than actually her, so none of these scenes are recreations of anything that happened in real life."

"We were both very cautious about how we were telling the story and the engine of the story," Bell said. "I just wanted to make sure that the joke was never about her or her body."

Brittany Runs a Marathon isn't afraid to portray the difficult reality of such a challenging transformation. At her lowest, Brittany lashes out at a complete stranger, Jasmine (Sarah Bolt), about her weight at a barbecue. "It showcases where Brittany's at mentally at that point," Bell said. "She hasn't grown, and she's projecting onto this woman everything that she feels, and it has nothing to do with this other person. A lot of people who don't work through their own demons look at other people who are happy in their lives and they have to basically put them down to make themselves feel better. "

Brittany Runs A Marathon - Official Trailer | Amazon Studios www.youtube.com

Bell also appreciates scenes like that, because they show the complexities behind overhauling one's life. Brittany can start a fitness regimen and lose weight, but there are still psychological habits she must break. "I think it was an important scene to show in this movie because a lot of times when people lose weight, people naturally think oh, now they're 'better,'" Bell said. "They don't work on the emotional journey, the mental journey that goes with that, and how one is related to the other."

Brittany's journey has ups and downs, but even in the darkest moments, she's moving towards the light.

"She's having a good time until the doctor tells her, 'You need to do something to change your life, because you're not healthy right now,'" Bell said. " Once you know that, where do you go? You either move forward, or you move back."

Colaizzo hopes audiences recognize their own human flaws in Brittany's behavior and strive for self-improvement the way Brittany does. "There's times in the film that you start thinking, 'I know that girl. Wait, I am that girl.'" Colaizzo said. "By the end of the film, [I hope you think], 'If I push myself hard enough, I can be that girl.'"

Brittany Runs a Marathon is in theaters Friday, August 23.