Improvisation is a hallmark of the modern screen comedy. In the films of the "Frat Pack" of Vince Vaughn, Ben Stiller, Will Ferrell and company, it's not the script that lands the laughs. It's funny actors riffing, trying to top the script.

Katy Mixon found that out working with Vaughn, his pal Jon Favreau and Reese Witherspoon in the holiday farce "Four Christmases."

Mixon plays the ever-pregnant rube-wife of the rough-and-redneck "cage fighter" Denver (Favreau). Watch the Taboo board game scene in which the bumpkins shock their city-slicker visitors (Vaughn and Witherspoon) with how in sync they are as a couple. Their victory at Taboo pays off with a kiss, and a little something extra - a slap.

"A little bit of a surprise," Mixon laughs at the "love tap." "I wasn't expecting it. It was supposed to say something about his character, about the relationship, I guess. And it's out there. The whole movie had so much improvisation that every take was something new, something different, trying to find the funniest version."

Mixon is a Pensacola, Fla., native who steps into the spotlight with "Four Christmases." To play Susan in the film, Mixon had to find her inner Panhandle, that part of Florida ("Florabama," she calls it) that's more like Mobile than Miami Beach. Her charming drawl, peppered as it is with "Honey" and "Oh my GOODness," made Mixon, 27, a good fit for Susan.

"You know what, honey? I made her up. Well, OK, I come from a family of nine and my mommy had seven children, so it wasn't hard for me to channel being preggers. And being pregnant during the holidays? Got to be really rough. The Southern part of it was the most natural of all."

Lest you think the rube turn is a bit close to home, Mixon is an alumnus of the famed Carnegie Mellon School of Drama. Of course, being distinctly Southern made her stand out there, her teachers say.

Now that she has Hollywood's attention, Mixon is mixing it up. She steps out even further as she's featured in the thriller "State of Play" with Russell Crowe, and co-stars in "All About Steve" with Sandra Bullock.

Her Southern-ness and sense of humor landed her the plum role of April, the old flame of baseball burnout/rehab candidate Danny McBride ("Tropic Thunder") in HBO's "East Bound and Down."

"When I saw Katy do a monologue from 'As You Like It' - a monologue one sees time and again - and realized she could make Shakespeare falling-down-funny in only a minute and a half, I knew we had a very special talent," recalls Don Wadsworth, one of Mixon's teachers at Carnegie-Mellon.

The presence of five Oscar winners in the cast (Mixon shares scenes with Robert Duvall and Witherspoon) drew Mixon to the movie - "Can you IMAGINE?" It's sure to be a hit, though the reviews were never going to be great. ("Occasionally amusing but thoroughly cheerless," Variety opines.).

For now, Mixon is homeward bound, remembering her own holiday nightmares, not wholly unlike the accidents in "Four Christmases."

"We had this lovely decorative wreath one year, I remember. Hung it right over the mantle over the fireplace. But my mom's really into candles. And, well, wreaths can catch fire."

She laughs.

"You know, I should've mentioned that on the set. I'll bet we could've used that!"

(c) 2008, The Orlando Sentinel (Fla.).

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