• When you screen it the first couple times, you're just trying to get the movie to work, trying to get the story to flow, trying to find out where your areas are where you have enough breath to laugh a little bit. So you're doing that the first two or three screenings, and then finally, you dial the movie in and it's working, and at that point, it's 50/50 as far as what's funny and what's working. Sometimes you'll put something in and it will just die so hard that it'll almost kill the movie.

    Adam McKay: When you screen it the first couple times, you're just trying to get the movie to work, trying to get the story to flow, trying to find out where your areas are where you have enough breath to laugh a little bit. So you're doing that the first two or three screenings, and then finally, you dial the movie in and it's working, and at that point, it's 50/50 as far as what's funny and what's working. Sometimes you'll put something in and it will just die so hard that it'll almost kill the movie.
    Source: film.avclub.com