Amanda Seyfried with her birthday cake.

She got famous from "Mean Girls." Now, Amanda Seyfried's ready for a very adult role: Playing porn star Linda Lovelace in an upcoming bio-pic.

"It's going to be really hard and kind of terrifying at the same time," Seyfried told ABCNews.com Tuesday. She was celebrating her 26th birthday at a party at New York's Norwood club hosted by the Clé de Peau Beauté makeup line. "It's something completely different. I do have some liberties there but I am playing someone that existed in history and had quite an established reputation for something very extreme."

Several actresses have been attached to the high-profile project. Lindsay Lohan was originally pegged to play Lovelace; Kate Hudson also circled it. Since landing the role, Seyfried said she's been doing a lot of research, and now, she's ready for action.

"I've read everything I can possibly read and now I just have to start physically getting into a mode," she said. "She was such a different person than what we expect. It's a good story to tell, and I'm really, really excited about it."