Graham: John Cleese
Both of them are the progenitors of great comedy groups. Both have directored and starred in their own films, and have unique comedy styles that are very hard to reproduce.
Paul: Randall Munroe
Although Munroe is not an actor, his comedy stylings of
www.xkcd.com just remind me of smart and witty things that Paul probably thinks about and keeps to himself.
Bill: Dennis Leary
Bill is an ass and hilariously funny. Leary is an ass and hilariously funny. 'Nuff said. Both are sinister.
Morgan: Sarah Silverman
Although of the opposite gender, Silverman captures Morgan's manly essence and utilizes it in female form. If Travolta can pull of an overweight lady, Silverman can pull off an overhorny man.
James: Neil Flynn
James reminds me of the character of the Janitor from Scrubs. People make fun of him, but he doesn't really care. I don't know how Neil Flynn actual acts, but he's the Janitor, so I'm sure he doesn't really care what I think.
Matt: Josh Peck
Sadly, I like Josh Peck. I think he's a great slapstick comedian and is often the butt end of jokes in roles that he plays, which is very similar to Matt.
Jer: Steve Carell
Both of them are funny because of their body language as well as their literal senses of humor. I think Carell could pull of a pretty good Jer.
Kathleen: Jeneane Garofalo
Both are straight talking women who speak what's on their mind, and have a great sense of humor.
Alex: Vin Diesel
Alex brought DnD to the LRR Forums, and his IRC game that I played once was pretty fun, which caused me to play it with my current roleplay group in real life, so I credit him to bringing me into DND. Vin Diesel has played DnD for over 20 years, so that's a big similarity right there. Both are big into roleplaying, and I'm sure Diesel could do Alex justice.
Alright, I know you guys want us to participate in the contests, but damn. This one took a lot of effort.
Edited once for aesthetic enjoyment.
Edited twice for Leary's inclusion.