It’s a long way from playing sweet little Gretl in The Sound of Music on the Players stage in Sarasota to starring in a horror/sci-fi/comedy called Detention that’s receiving attention on the film festival circuit and just opened nationwide. But that’s the journey 20-year-old Shanley Caswell, a Sarasota native, has made in just a little over a decade.

Caswell, a self-described “shy kid,” fills up the big screen right now in Detention, co-written and directed by veteran music video director Joseph Kahn. She plays a high school senior named Riley, a cynical, smart, witty social outcast who’s one of a group of teens being terrorized by a serial killer dubbed Cinderhella. Her costars include Josh Hutcherson (currently being seen in the film version of the popular Hunger Games books) and comedian Dane Cook as the school’s principal. But for the first time, Caswell plays the leading role—an experience she says was both the toughest and the best she’s had in her acting career so far.

“It’s a low-budget indie movie, so that meant long hours while still doing school, and not sleeping a lot,” explains Caswell (interviewed from the North Carolina set of the movie she’s filming now—more on that in a minute). “Plus it was really hard work—lots of screaming, crying and running. But everyone on set was excited to work there. We were all young and good friends and there was a lot of kinetic energy. “

The starring role in Detention (which won the Youth Jury Prize at the 2011 Seattle International Film Festival) came after an audition for director Kahn, who says he must have looked at a “couple hundred actresses” before finally deciding on Caswell. “I didn’t know who she was initially,” he says, “and she came in—late—the first day of auditions. She just nailed it; she was so natural, so funny. But I put her away in the file because, quite frankly, I was looking for a ‘name.’”

He didn’t find one, but, Kahn adds, “If we had to hang the movie on someone’s shoulders, it was going to be her. I actually kind of liked that it would be someone unknown. As it turned out, Shanley is not only natural, but she’s technically good, too. And our stunt coordinator on the film, who has done Jackie Chan movies, was impressed with what she can do; she’s a natural athlete. She’s the complete package. Plus, in a kind of crazy movie, she’s the real person at the center of it who keeps us involved.”

Who could have known how far she’d go when Shanley first took to local theater stages? Sure, her parents (Hermitage Artist Retreat program director Patricia and attorney Chris) had been onstage in theaters in town themselves, so it seemed natural enough for Shanley to join them. (Brother Ben did some acting here as a kid, too.) And Shanley recalls enjoying doing the Players’ youth productions while still an adolescent.

But it was when she starred as Scout Finch in the Asolo Rep’s To Kill a Mockingbird, she says, “that it became a job, and everyone took it seriously. It was the first time I felt responsibility to a role and really connected with a character.”

She was hooked. And luckily, through friends she met who were also actors, she found a manager who encouraged her to come to California and give film and television acting a try. Although she was still in high school, Shanley and her parents decided she could leave home and live with good friends in Los Angeles.

“I liked the energy there,” Shanley recalls. “My family and I were questioning if I should do it, but once I was on a set I realized I really wanted to give it a go. At 16 I graduated with my GED so I could work longer hours. And I started taking college classes in Santa Monica while staying with this really grounded family who rented space in their home to all these kids who worked for the Disney Channel. I felt part of a family there; I never felt isolated.”

Once settled in California and making the audition rounds, Shanley says, “There was never a big breakthrough moment. It was just gradual. I got some commercials, some guest spots [including on Nickelodeon’s Zoey 101], and then small roles started to get bigger. My first big project was the Hallmark movie Mending Fences, where I worked with Angie Dickinson.” That got enough attention to warrant her move to a bigger agency—another stepping stone.

The drive that’s led her to this point in her career means, she admits, that the average teen social life has definitely taken second place. But acting hasn’t kept her from continuing with college; she’s studying cultural anthropology at UCLA, with less than two years to go for her degree. “I just wanted to read and write a lot,” she says of her choice of a major. “It helps both in everyday life and as an actor, to open your mind and explore other worlds. Some actors tend to get focused on just acting, and they get very involved in the movie premieres, the glitter, the spotlight. That’s not what I’m doing this for.”

In her current film project, tentatively called The Warren Files, she plays the oldest daughter of a family that innocently moves into a house where paranormal activity is taking place. (Patrick Wilson, currently starring on TV’s A Gifted Man, and Vera Farmiga, acclaimed for her work in the film Up in the Air, play the parents.) That’s pretty lofty company, and a sign that her career is on the rise.

But Shanley isn’t letting that go to her head. “It’s a little too early to tell,” she says, whether acting is a long-term career. “I enjoy it; and right now, I’m just going with the flow.”