23. Jimmy Fallon (1998-2004)
Prior to his talk show days, Jimmy Fallon used to catch a ton of heat for busting out laughing and breaking character in the middle of his sketches (a crime which scores him a beating from Peter Griffin in an episode of Family Guy), but we can't fault the guy for it. Imagine trying to keep a straight face while acting out "The Barry Gibb Talk Show" with added chest hair and crazy gold medallions or the "More Cowbell" sketch, with Will Ferrell flailing around in a belly shirt. It just can't be done.
Fallon's laughter always proved infectious, and often made us (and sometimes other cast members) crack up in the midst of scenes that otherwise weren't that funny. That said, the goal of the show, first and foremost, is to make people laugh. It's only natural the actors give in from time to time. Besides, it made filming the show look like a blast.