Everyone is not thinking this through fully. When you say a well-known actor who may be 'unconventional', we're not talking about black actors, or oscar level winners, such as Jamie Foxx or Leonardo DiCaprio. The fact is that most people in the lists I've seen are big actors who WOULD be conventional for this film, because they've done genre stuff before.
DiCaprio - Titanic, The Beach, The Aviator, The Departed (It's not THAT hard to see him play a comic-based character following these larger than life roles).[/i]
Jamie Foxx - Ray, Collateral, Miami Vice - Like DiCaprio, he's played roles which may him less conventional.
Damon - Bourne Series, Ocean's Series, The Departed - Far too many range of roles, and both critical and mainstream success make him fit the role.
There's 3 groups of actors that would be 'unconventional', and both would be VERY controversial to the fans:-
(1) The MOVIE STARS - Pitt (he's not conventional due to his name, not the roles he's played), Cruise (NO), Hanks (?), Smith (Hell, NO!), Gibson (too old), Washington (maybe...)
(2) The Well known/non dramatics - COMEDIANS - Will Ferrell (face it, NO-ONE could see him in the role), Stiller or Wilson (F@!K NO!!!), Vaughn (???), Paul Giametti (VERY unconventional, but a bloody good actor), Carrell (Nah...), Lawrence (NOOOO!!!!!).
(3) Non-actor Actors, fan-driven - FACTOR X - Bowie (?), Serkis (hmm...), Ice Cube, Eminem (No, sorry...).
I'm REALLY hoping it's somebody that's been mentioned by any sites so far. I say to everyone to think OUTSIDE the box.