The American Wedding star talks about her chemistry with onscreen costar Jason Biggs and offscreen fiancé Alexis Denisof.

08/08/2003 AT 01:55 PM EDT

Band camp has never been the same: Alyson Hannigan has become a fan favorite both on the big screen (as American Pie's nerdy-but-randy band geek Michelle) and on television (as Willow on Buffy the Vampire Slayer). Now she's back on top with the current No. 1 movie American Wedding – the supposed final installment in the Pie trilogy, in which her character, Michelle, ties the knot with the once-pastry-loving protagonist, Jim (Jason Biggs).

In real life, Hannigan, 29, is prepping for another American wedding: her marriage to Angel costar Alexis Denisof, 37. Hannigan tells PEOPLE about her engagement and her return for another piece of Pie.

What is it about Jim and Michelle that makes moviegoers interested in them?
I think that there's a bit of the underdog quality to them. You sort of root for them to succeed because they are trying so hard – or at least Jim is. And Michelle is just quirky. She doesn't really care about everybody's perception of her ... which is also really a sweet quality.

Do you have a favorite big-screen couple?
Billy Crystal and Meg Ryan in When Harry Met Sally.... They had amazing chemistry in that movie, not to mention a brilliant script and great direction.

Do you and Jason Biggs have a similar chemistry?
Oh, I would never compare myself to such higher-ups. But I aspire to be in that realm.

What about your real-life engagement to Alexis Denisof? How did the proposal go down?
I think I did have food in my teeth, so it was a little awkward. We were driving up to his mom's house for Christmas. It was this wonderful road trip and we were up in Sonoma and Napa (Calif.). We bought all these beautiful foods and Alexis found this lake pretty close by, so we went to the lake. I'm swallowing the food, and then he just started saying these beautiful, wonderful things. The whole time I am getting kind of frustrated, like, "Why does he always do this? He says the most romantic things and it would be the perfect way to propose and then he doesn't propose."

So then at the end of it, he asked me to marry him and I was like, "Huh?" Everything in my brain just screeched to a halt. I wish I had TiVo, so I could rewind the whole thing.

So it was a surprise?
It was a complete surprise. But I knew he meant it because he had a ring.

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