Sometimes actors inject reality into a scene in which they are drunk by ingesting a bit of alcohol. Or a lot. Acting while drunk is risky, because there's a chance an actor may forget his lines, but the best actors will either remember their lines or improve upon them as they go. And on rare occasions, shooting an actor while drunk leads to new footage that wasn't in the original vision for the film, as in Fight Club.
Read about more such scenes, with video evidence, at Unreality. Link
The rumor was that Brad Pitt and Edward Norton were firing golf balls at the catering truck after they had been doing some late-night drinking on the set. Fincher th0ught it was so cool and anarchistic of them, he turned the camera on them and had them do the dialogue for the scene while hitting said golf balls at said truck while under the influence of alcohol.
There is such an authentic feel to everything about Fight Club, and that is because everything, right down to the drunken alpha-male chemistry between its stars, WAS authentic.
Read about more such scenes, with video evidence, at Unreality. Link