Rose Byrne plays a mother in the midst of a breakdown in the experiential psychological thriller “If I Had Legs I’d Kick You.”

Anticipation was high for the A24 film, which will be released sometime this year. Its premiere Friday at the Sundance Film Festival was easily the hottest ticket in town, with even ticketholders unable to get in. Those who did make it into the Library theater were treated to an intense, visceral, inventive story from filmmaker Mary Bronstein that has quickly become one of the festival’s must-sees.

Byrne plays Linda, who is barely hanging on while managing her daughter’s mysterious illness. She’s faced with crisis after crisis, big and small — from the massive, gaping hole in their apartment ceiling that forces them to move to a dingy motel, to an escalating showdown with a parking attendant at a care center. The cracks in her psychological, emotional and physical wellbeing are become too much to bear.

“I’d never seen a movie before where a mother is going through a crisis with a child but our energy is not with the child’s struggle, it’s with the mother’s,” Bronstein said at the premiere. “If you’re a caretaker, you shouldn’t be bothering with yourself at all. It should all be about the person you’re taking care of, right? And that is a particular kind of emotional burnout state that I was really interested in exploring.”

Byrne and Bronstein went deep in the preparation phase, having long discussions about Linda with the goal of making her as real as possible before the quick, 27-day shoot. Byrne said she was obsessed with figuring out who Linda was before the crisis. The film was in part inspired by Bronstein’s experience with her own daughter, but she didn’t want to elaborate on the specifics.

“That’s her story to tell,” Bronstein said.

Part of Linda’s story involves her therapist, played by Conan O’Brien, who joked that he didn’t realize he was in a movie.

“I’m not looking out for movie scripts or anything. But when I got a call from A24 that they wanted me to read something, I’m not stupid,” O’Brien said. “I showed it to my wife, who is one of the smartest people I know, and she read through it and she said, ‘I didn’t know they made movies like this anymore.’”

He was particularly in awe of his director and co-star, saying he felt like a fraud standing beside them.

“It was an amazing experience, one of the best experiences of my life, just to be with them and watch them work,” O'Brien said. “I don’t know how (Byrne) did that and not check into a hospital afterwards, because I haven’t seen any actor, man or woman, sustain that level for an entire movie.”

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“I feel like I have to go to a hospital now, because this was the first time I watched it,” he added. “I’m a mess.”

A$AP Rocky also co-stars, as a man Linda meets at the motel, but was not in Park City for the premiere. He is currently on trial, charged with firing a gun at a former friend.

The film is full of ambiguity, metaphor and just plain artistic expression that Bronstein hesitated to explain, from the name itself to the hole in the ceiling, which takes on a somewhat supernatural quality.

“When we have nothing left to give, we have an emptiness inside of us,” Bronstein said. “And that emptiness is actually not empty: It’s filled with all the darkness and self-hate and doubt and fear and dread and regret and everything. … That to me is what the hole is.”

Some of it, she said, she doesn’t even fully understand. The point is the experience, and critics and Sundance audiences are already fully on board.

Bronstein, a bit of a cult figure in the film world, made her directorial debut in 2008 at the SXSW festival with “Yeast,” which featured a pre-fame Greta Gerwig and was hailed by by New Yorker critic Richard Brody as a “mumblecore classic.”

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“If I Had Legs I’d Kick You” is only her second feature.

“This is the first time that anybody else has paid for me to make art,” Bronstein said. “I’m proud to say that this is the film that came directly from my head to the screen.”