Though she's been acting for 15 years, 21-year-old Kaley Cuoco faced a nearly insurmountable challenge when she agreed to star in "To Be Fat Like Me," a Lifetime Original Movie premiering Monday.

She plays a high school student who goes undercover as a "fat girl," donning prosthetics to increase her girth to an ungainly 250 pounds. This is a real stretch for Cuoco, who's been a trim jock all her life. She was a nationally ranked tennis player and is fiercely competitive in her spinning and kickboxing classes.

Cuoco, whose dad is in beachfront real estate, grew up near Santa Barbara, about 100 miles north of Los Angeles. But she says her family kept moving closer to L.A. as she became more engrossed in her career.

She was making "8 Simple Rules ..." and driving an hour to work when she decided she needed to vacate the nest, though she was only 17.

"I got my first condo near Disney in Studio City and that was it, I was on my own. I didn't think I'd be scared, but ... I cried the whole first night. My parents live 25 minutes from me, and I'm there more than I'm home. I'm there more than my sister is, and she lives there. I have the best of both worlds."

She grew up in a conservative Christian family and says that background has helped her overcome the difficult times, including preparation for this role.

It took 3 1/2 hours to apply the prosthetics.

"There wasn't a day when I didn't cry. Your mind plays tricks on you. You start to think, 'Oh, my God, I can't get out of this.' You can't move. You can't breathe. Then you start panicking and you have to really find that place. I had to go to a different place each time I was in that suit or I would've completely freaked out," said Cuoco.

"The first night I was in the fat suit the whole day, I called mom and was sobbing, crying. I said, 'I can't do it.' She just was a mom: 'Tomorrow you'll be fine.' "