LOS ANGELES — "They are such the couple that belongs together," says Alyson Hannigan, lounging on a makeshift patio outside the set of CBS' "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 closes the comedy's second season. The show centers on five 30-something 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.

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."

"I just love comedy and wanted to focus on that," she says.

"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. 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.

Creators Carter Bays and Craig Thomas hired Hannigan without her even reading for the role.

"We just met with her and got to know her as a person," Bays says between scenes, the scent of lilies and roses from the mock wedding hall wafting through the set. "Almost the first thing that we looked for was, 'Is this the kind of person we want to hang out with?' "

At 33, Hannigan is an entertainment veteran, having started out 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 ..."