As she prepares to leave her teenage years behind, 19-year-old Amanda Bynes has been undergoing some substantial -- and very unusual -- changes.

"I was sitting in a trailer with my mom, and I'd be shoveling food in my mouth like a man, because I felt like a man," the "What a Girl Wants" star recalled. "I started feeling not feminine, and I wasn't getting any attention from cute guys whatsoever."

One of Hollywood's most promising young comedic actresses, Bynes has shocked and confused her fans with the "She's the Man" trailer and accompanying photos, which show her sporting frumpy men's clothing, sideburns and even a mustache. (Watch Bynes transform before your eyes right here.) But Bynes wants to set the record straight: The only place you'll see her new, manly appearance is on the big screen.

"I was never thrilled with the idea," she said of her role in the comedy, due March 17. "When I was on 'The Amanda Show' when I was a kid, I became so many different characters every week. I was 14, and I was just starting to want to look pretty and be a cute girl because I liked all these guys. But when I read the ['She's the Man'] script, I just felt it was so funny."

The plot that convinced Bynes to set aside her vanity revolves around Viola, a tomboy stunned by her high school's decision to eliminate funding for the girls' soccer team. When her twin brother, Sebastian, goes on an extended vacation without telling anyone, Viola comes up with a wicked idea: Why not dress up like Sebastian, attend his new school and show everyone that a girl can play on the boys' soccer team?

Previewing an exclusive clip from the movie, Bynes explained Viola's big transformation scene. "I really feel like it's the montage moment, like in 'Mrs. Doubtfire,' when [Robin Williams is] getting made up with all the different looks," she said, remembering all the mustaches she tried on. "I cried, I was laughing so hard. They were all fun, but we just didn't think it would look realistic to have a mustache. Which kid in school really does?"

Watch an MTV exclusive clip from "She's the Man" in Overdrive

Of the sequence that follows, with Bynes observing the manly mannerisms of random pedestrians, the actress said, "That is actual footage. I was walking along a very busy street in Vancouver, and then some people would get freaked out and angry at me, but it was fun."

People occasionally turned around with a spark of recognition, wondering why Amanda Bynes was taunting them. "I would just be like, 'Act casual, we're making a movie,' " she laughed.

The finished product of the transformation is less a studly dude than a Doogie Howser type -- an underdeveloped, awkward, wise-beyond-his-years teenager who ends up seeming more realistic than the beefy, twentysomething actors typically cast in high school flicks. "Part of the thing that's funny about it is that I do look so young," Bynes said of the real-life physical attributes that made the makeup team's job easier. "I really did end up looking like a guy that people have gone to school with, like that nerdy kid."

Bynes says many people have pointed out another familiarity in the post-transformation Viola: a resemblance to Jesse McCartney.

"When they were convincing me to do this initially, I said, 'Why do you want me to play a man?' " she recalled. "They actually took a photo of me, and then morphed it with Jesse McCartney. It was so damn convincing that I was like, 'That's kinda cool,' and we saved it. I was like, look at what I'd look like if I morphed with Jesse McCartney."

The filmmakers even turned to the singer when casting the role of Viola's twin brother. "They actually tried to get Jesse McCartney to do it, but he was doing his show at the time," she said. "We lucked out that we found a guy [who had] oddly similar features."

It was a good thing they found actor James Kirk, Bynes reasons, because the "Beautiful Soul" singer might not have been too excited about the comparisons. "They morphed him with a lady," she laughed, "so I don't know how thrilling that would be."

While her scenes as a guy have garnered the most attention, Bynes says playing Viola in "She's the Man" permitted another cinematic transformation. "People have said to me, 'Wow, Amanda, this is more of an adult role,' " she said, citing moments in the film in which she wears tight dresses and high heels. "I don't think people have realized that I am 19 now, and almost 20. I've grown up a bit."

Making plans to enter her third decade in style, Bynes expects to be out of the country for her birthday on April 3. "I'm psyched," she said excitedly. "I think I might be in Australia for the movie. So I might have a few shrimp or something."

One important party detail has already been planned -- she'll be attending in women's clothes. "If there's a choice with that or being in a man's outfit, I'd be like, 'Can I please dress up as a girl?' " Bynes laughed. "Because I was a man throughout most of ['She's the Man'], the times that I can be a girl, I'm like, 'More makeup, please.' I think it's my ego wanting a little more caressing."

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