"What if the mom comes on to me?"

The credits say Step Brothers was written by Adam McKay and Will Ferrell, but talk to McKay -- who directed the goofball dysfunctional family comedy about two grown guys (Ferrell and John C. Reilly) still living at home with their parents -- and he'll tell you that practically every scene was improvised.

"There was a load of improv on this one, way more than any film we’ve ever done. But it was built to hold that," McKay says. "The weird thing is, we'd go through our script, we'd rewrite it like 15 times and we'd do three read-throughs, so by the time we hit the stage that script was tight. But, man, the stuff we find out in the improv is always so good, so I would usually do three or four as written, and then do four or five improvs.

"There were days when the actors would come in exhausted and say, `Really? We’ve got to make up whole scenes?'

"`Yes you do, lets go.'

"We shot an obscene amount of footage on this movie. The DVD is going to be packed with material. It's the most improv I’ve ever done on any project I’ve ever worked on, including the show I did at Second City, which was based on improv."