So Katie Holmes ran the Boston Marathon. Big deal. You want to hear about a feat of endurance? I just transcribed an interview with Vince Vaughn. Word for word. To quote Ringo Starr, I’ve got blisters on my fingers!

You don’t always notice this because his voice isn’t loud—it’s rather soft and mellifluous, with a tendency to drag out his “a”s—but Vaughn can talk a mile a minute. Here’s how New Yorker film critic Anthony Lane described Vaughn in the 2005 hit The Wedding Crashers:

“Vaughn, who seems to grow taller with every film, [is] like a hopped-up Gary Cooper…who shoots his mouth off with such scattershot brio that he flubs the occasional word, or picks an overheated one, or forgets to pause for breath…[he] is either a genius or an escaped lunatic who should not be approached without a stun gun.”

This might not seem to be the sort of talent one would expect to find in a family Christmas movie (a phrase that used to be redundant until Bad Santa), and yet here Vaughn is starring in Fred Claus, which opens this Friday. Reunited with Wedding Crashers director David Dobkin, Vaughn has the title role of Santa’s older brother, a good-hearted soul who nonetheless hasn’t been able to find a niche in the world.

As the film opens, he’s working as a repo man (Santa gives, but Fred takes away) and saving money to open an OTB parlor. Because he’s resentful of his brother Nick’s success, he’s been estranged from the family. But circumstances conspire to bring Fred to the North Pole to help out during the December crush—at the same time when Santa’s operation is threatened with being shut down by a corporate cost-cutter.

Such is the plot, and you’d be well advised not to start unraveling it, as the knots don’t hold. Still, it’s a family lark with all the special effects you’d expect from a Joel Silver production and a surprisingly strong cast: Aside from Paul Giamatti, who makes for a wonderfully stressed and neurotic Santa Claus, there’s Kevin Spacey, Kathy Bates, Rachel Weisz, Miranda Richardson, John Michael Higgins and Chris “Ludacris” Bridges.

Vaughn, who got his start doing improv at Chicago’s Improv Olympic theater, is a great believer in using improvisation to flesh out his parts. At a recent press junket on the Warner Brothers Studio lot in Los Angeles, amid recreations of the movie’s North Pole sets and surrounded by some of the Russian circus performers who plays elves in the movie, he discussed one of the scenes that wasn’t in the original script, an early confrontation between Fred and a young girl who doesn’t like the fact that he’s repossessing the family TV.

“We wrote that that day,” he recalls. “This movie was tough because we didn’t have the normal prep time that we had to write on something like Wedding Crashers. The writers already had a lot of great stuff in the script. But it’s our process to go through it and do material. And that’s a situation where I was able to improvise and push the limit knowing that David Dobkin would go and make sense of the whole thing [in the editing room]…That’s not always the easiest job, sifting through all the dirt to find the gold.”

While Spacey and Giamatti admit that they were initially a bit nonplussed about the idea of moving away from the script, they came around to Vaughn’s way of thinking. “It’s all very childlike and make believe,” he feels, “that’s where it came from for me. There shouldn’t be such a religious approach to the arts where it’s sacred in some way. [Improvisation] is just opening a door and being given that opportunity.”

The great thing about improv, of course, is that you can bring just about anything else that interests you to play on what you’re doing. For a scene in which he has to persuade the North Pole’s DJ (Ludacris, shrunk to elvish stature via digital technology) to play something—anything—besides “Santa Claus Is Coming to Town,” Vaughn drew on one of his favorite TV shows.

“I like that show Supernanny—you ever see that? This Englishwoman is unbelievable! These kids are hopped up on Pop Rocks and Coca-Cola, they won’t listen to anybody, but then the Supernanny comes in and sets boundaries and all of a sudden they start listening. Sometimes I’m left alone with [my niece and nephew], and I don’t want to be the uncle that has to discipline them, you just want to have fun with them. But when they start hitting each other you have to put down some kind of law. So I try to Supernanny them.

“So when we were doing the thing with Ludacris, I just started talking like Supernanny.”

Though at 6’5” Vaughn towers over the movie’s substantial cast of elves, he’s clearly in touch with his inner child. He admits to being a big fan of Christmas, to the point where he’s already working on another Christmas movie for next year.

“It’s called Four Christmasses,” he explains. “It’s about people that come from divorced families so you’re forced to go to four Christmasses. It’s about the kind of stress that can come if your parents re-married people that maybe you don’t necessarily get along with.”

In the meantime, he’s at the age where he can rediscover Christmas through his nieces and nephews. “When I was a kid and you’d get gifts, you’d feel the package and you could tell it was socks, and you’re like, ehh, I don’t want socks. And you’d feel the next package and it would be jeans. I was so painfully skinny anyway that it just a reminder of how awkward I felt in the first place. But then you’d get toys and that was great.

“But then you get to a certain age and all you get is clothes, so the fun of it is kind of over. But now that fun is back in it—there’s nothing like kids at Christmas time. The joy that they get getting something they’ve been asking for, I’m not real good at setting stuff up, but that’s the best time to connect with kids.

So does he think he’d make a good father? “Here’s the thing that’s advantageous about being an uncle: I get to play with them, I get to say they’re great, and then I get to go home. I think I would be a good dad once they got be six or seven and I could communicate with them. Being around them in those early stages, you know, when they’re crying, there’s only a few things they could possibly want, but it’s figuring out which one of those things that seems like an eternity.

“My nephew who is four has seen the billboards for Fred Claus, and he recognizes me—‘Uncle Vince! And that’s Santa Claus!’ And my sister is mortified because she doesn’t want the concept of Christmas to be ruined for him. But the thing he was upset about was, ‘Uncle Vince knows Santa Claus, how come he hasn’t introduced me?’”

Inevitably the question arises, what would the cast of the film ask for if they had one Christmas wish? Vaughn co-produced Fred Claus, so his immediate answer makes sense: “A sequel!”

But when Giamatti says he’d ask for “Peace on Earth,” Vaughn refuses to be chastened. “Yeah, that makes sense,” he says, “I think peace on Earth is something good to say too. Here’s where I would go: hot tub, a couple of girls from Brazil and a ‘do not disturb’ sign. And peace on Earth.”