Hilary Duff

John Cusack & Hilary Duff's Mutual Admiration Society

By Mary Margaret and Emily Fromm

06/13/2007 at 02:40 PM EDT

Hilary Duff had an ultimate fan moment when she saw John Cusack in L.A. on Tuesday.

Duff, 19, was at the W Hotel's Whiskey Blue bar for the party following the premiere of Cusack's new horror movie, 1408.

"She walked in holding hands with her sister Haylie," says a fellow partygoer. "But the moment she saw John, she ran up to him gushing about how much she loved the movie and him.

"She shrieked about how scared she was and even covered her face to reenact her horror. It was so cute!"

The admiration, apparently, is mutual. Cusack, 39, who costars with Duff in the upcoming satirical action movie War, Inc., recently told MTV she was "a revelation."

He'd requested her specifically for the role, he said: "The part was ... she played a pop idol from central Asia. So, she was like a Hilary – but not Hilary, because Hilary is so classy – but like a real, and I won't name names, but a real slutty pop star. ... Hilary is the opposite of that, of course. I think for her to play that, or to parody that, was really great."

Asked what is the biggest misconception about Duff, he gushed: "I think probably people wouldn't know how talented she is. I mean, she's a great actress. I just spent the entire fall with her and she was a revelation every day. I don't think people know that yet, but they're gonna."