Kate Mara Says Stepson, 11, Is 'Biggest Fan' of Her Film “The Astronaut”: 'He Was Freaking Out the Entire Time' (Exclusive)

Kate Mara's new movie The Astronaut has one very special fan.

In an interview ahead of the sci-fi thriller's SXSW Film & TV Festival premiere, the actress, 42, told PEOPLE she watched the movie for the first time with her 11-year-old stepson.

"I brought my eldest kid with me to see the film for the first time, I hadn't seen it yet. I brought him with me in a pretty big audience, and everyone was on the edge of their seats," says Mara. "I would say it's a very thrilling experience. Feels very much like a roller coaster. You feel like you're on a ride."

"He was freaking out the entire time," Mara adds. "He kept saying, 'Tell me what's going to happen, tell me how it ends!' the whole time, literally from the very first scene. He was like, 'Okay, what's going to happen next?' But he loves it. He has an astronaut jacket that he wears out every day. He's the movie's biggest fan."

In The Astronaut, Mara plays Sam Walker, the titular astronaut who, upon returning home from her first space mission, starts witnessing otherworldly occurrences while staying under NASA's care at a remote, high-security safe house. Mara stars alongside Laurence Fishburne and Gabriel Luna.

Related: Kate Mara Teases The Astronaut, Her 'Thrilling' New Alien Thriller with Laurence Fishburne (Exclusive)

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Writer-director Jess Varley adds how "sweet" it was for Mara and her stepson, whom Mara's husband Jamie Bell shares with ex-wife Evan Rachel Wood, to share that moment together. (Mara is also mom to two younger children with Bell, 38.)

"We went over for dinner not that long ago and he was like, 'I have a ton of questions,' " Varley says, with Mara noting that despite her being the lead of the movie, she thinks her child "understands the movie far better than I ever will."

Mara says a "highlight" of the project was sharing scenes and bonding time with costar Fishburne, 63, who is also an executive producer on The Astronaut.

"He is the greatest storyteller ever," Mara says. "We had never met before, but by the end, any time in between scenes or setups, it didn't matter how long we had, whether it was five to an hour, we would sit in the green room and just talk about acting."

"It was so incredible to work with him because he has such passion and such joy and love for acting, still today. He's been acting since he was a tiny kid," she continued. "I feel very much like that as well, and I've been acting since I was little too. To see somebody that I really look up to who really still feels the joy and really shares the joy with everybody, it's so amazing to see."

As for how she handled any alien scenes, Mara explains, "I was basically looking at nothing. Maybe there was a tennis ball at some point, or I think we used a tree branch maybe at one point as well."

"I had to use my imagination and really rely on Jess and her vision," she adds. "If I was ever struggling with what I was meant to be seeing, I really felt like I could rely on Jess because her vision has always been really specific and clear, which is so useful. That's not always the case."

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