In the remake of the 1976 thriller, Stiles and Liev Schreiber play parents who slowly realize their little boy is the anti-Christ.
"I think the inexperience of not having been a mother actually helped," Stiles told UPI in New York recently. "Being a young mother in this helped because I think anybody who is more mature and had more experience with motherhood wouldn't have been so doubtful of herself, and wouldn't have been so confused about her relationship with her son. It was easy to imagine that that would be difficult."
The "Save the Last Dance" and "O" actress also said she was grateful to have the chance to show she has matured past the roles that made her famous.
"I'm 25, so it would be ridiculous if I was playing a teenager right now," Stiles laughed. "But, in terms of what the character has to go through, she was more sophisticated (than women I've played before.) I'm really happy with a lot of the movies I've done, but it's a natural sort of growing-up period."