LOS ANGELES - "They are

such

the couple that belongs together," says Alyson Hannigan, lounging on a makeshift patio outside the set of CBS's

How I Met Your Mother

. She's musing over a just-filmed wedding scene involving her character, Lily Aldrin, and Marshall Eriksen, played by Jason Segel.

"I love the fact that they are in a healthy relationship on television," Hannigan says. "I'm sort of sick of watching those couples where you're like, 'Why would they ever be together? They hate each other.' Maybe it's funny, but it's so tiring."

The two-part wedding episode beginning Monday (NBC3, 8 p.m.) closes the comedy's second season. The show centers on five twentysomething singles in New York, with Lily and Marshall its most stable twosome.

Of course, as it is with most weddings, televised or otherwise, more than a few things go awry on the big day.

"Look, I'm not going to stress out about every little thing," says Hannigan's Lily during a scene on Twentieth Century Fox's Stage 22. "I'm marrying Marshall today, and nothing can ruin that."

Fateful last words in sitcomland, of course.

"I definitely related to the whole 'OK, I'm not going to stress out,' " Hannigan says, recalling her own nuptials three years ago to actor Alexis Denisof, a ceremony not nearly as chaotic as the one she's doing on TV.

"We had our perfect wedding," she says. "Nothing major went wrong. Not that I knew of. But the wedding planner probably had a coronary a couple of times."

Panic becomes palpable, and wildly hysterical, as Lily - in and out of a long white wedding gown - attempts to thwart the inevitability of Murphy's Law.

It's a classic comedy of errors, and these days Hannigan couldn't be happier falling flat on her face - yes, literally - after six years of taking on demons in Buffy the Vampire Slayer.

"You name it, we did it on that show ... which was phenomenal," says Hannigan, who pursued sitcom work after Buffy because an hour-long show was just too demanding.

"Also, I just love comedy and wanted to focus on that," she says, relaxing on a sunny afternoon in a jean mini and layered long-sleeved T-shirt.

"I really responded to the fact that in the script for the pilot episode, you not only got to be funny, but also played real emotions," she says. "I still have that side that loves drama and loves to go there."

But it is Hannigan's deft timing and smooth comic energy that have some drawing parallels to another CBS leading lady: Lucille Ball.

"Been reading my mom's letters, haven't you?" she says with a laugh, humbled by the comparison. "I've always admired her and I could watch Lucy for hours. But I don't feel like that's true. I've always loved her and Carol Burnett. I'm sure that shapes how your sense of humor develops. But do I think I hold a candle to them? No, no, no. Talk to me in 40 years."

At 33, Hannigan is already an entertainment veteran, having started in commercials at age 4.

"People often ask me, 'What would you have done if you hadn't been able to do this?' I just can't come up with a good answer, or a truthful answer, because I just don't know. ... I would watch The Brady Bunch and want to be Cindy Brady. I just wanted to be in that world," says Hannigan, who now has more than two dozen movie and TV credits (including the American Pie film romps).

She even has two songs recorded about her: "Alyson Hannigan" by Juvenile Wreck and "This One Time," which borrows her now-famous American Pie line: "This one time, at band camp ..."