This is a two-parter:
What performances by a (standup) comedian in a movie have impressed you the most? The more dramatic the better, though they will no doubt have injected some (or much) humor into the role.
Which comedian is the best dramatic actor?
Bill Murray in Groundhog Day. I thought he showed a much better range in this than in The Razor’s Edge and Lost in Translation.
Robin Williams in The World According to Garp was IMO better than in Awakenings.
Jim Carrey in The Truman Show. I could never get the motivation to watch The Majestic; was he any good in it, I wonder?
Steve Martin in Roxanne. I never saw Grand Canyon either.
Adam Sandler in Punch Drunk Love.
I would probably give the nod to Bill Murray for the best dramatic actor of all comedians. In addition to the roles I mentioned above, he was good in The Royal Tenenbaums and Rushmore, and I look forward to seeing The Life Aquatic.
I’m sure this thread has been done before, but new movies (and comedians) come out all the time, so there might be new additions to the list. Also, I couldn’t think of any women to put on the list. There has to be at least one. And the role doesn’t have to be lead. As an example, Bill Murray in Tootsie. (There he is again!)