Polymath's picks:
The Comedian: George Clooney
Ozymandias: Owen Wilson
Rorschach: Kevin Spacey
Nite-Owl: Russel Crowe
Dr Manhattan: Ralph Fiennes
Silk Spectre: Jennifer Connelly
Mojo's notes/picks:
I don't think George Clooney could pull off Comedian's inherent brutality. I actually think Russell Crowe would do a better job as that character -- he can pull off brutal, and still be sympathetic. (One of the reasons I think Crowe would make a good James Bond...) In the book the Comedian represents the Perfect Man of the 1940s and Cold War: big, strong, aggressive, amoral, hard-working, pragmatic.
Another switch -- I'd put Ralph Fiennes in the Ozymandias role instead of Owen Wilson. Wilson has never played a serious role. He's always a goofball. When Fiennes walks on the screen we instantly like him and we think of him as intelligent and likeable. It was those qualities that were part of the reason he was cast in -- and so effective in "Quiz Show." He has also demonstrated an abililty to turn all that on it's head, and portray someone truly menacing. He's capable of mastering the complexity of the character. Ozy represents the Perfect Man of the Millenium: intellectual, precise, measured, Politically Correct, understated, ambitious.
Kevin Spacey is a good pick for Rorschach. He's done dead-pan narrative before. He can handle Rorschach's dark sense of humor. Menace and humor are the keys to the character -- it's why he's so well-liked by fans, despite the fact that his zealotry renders him completely incompatible w/ the overwhelming majority's sense of reason. If you met Rorschach you would WANT him to be taken of the streets, yet you root for him in the pages of the book. That's what makes him challenging to portray. Another possibility -- Don Cheadle. He's just as brilliant as Spacey at portraying complex likeable/hateable characters. Or perhaps Mel Gibson -- no one does crazy quite like Mel... (One downside -- Gibson might be too charming.) Watching Rorschach should be like watching a car accident -- you don't want to look at him but you can't help yourself.
The other characters are less complex, even the unworldly Dr Manhatten. He could be portrayed with regal serenity by almost anyone. Well, anyone who is tall, around age 30 w/ an absolutely flawless body who can act. It's important to the character that he be inhumanly perfect, physically, and that would be hard to find! The other characters could be played by actors who look quite different from the characters in the comic, but not Manhatten. Manhatten should come off as detached, most of the time, like he's just barely managing to pay attn.
Nite Owl is a straightforward likeable good guy. The biggest challenge w/ his character is that his lack of confidence w/ women must be charming, rather than annoying. Points to Hugh Grant... if he could pull off the US accent. But he's not my choice for the role. Just someone who can pull off awkward charm like he does.
***SPOILER ALERT***Silk Spectre must beautiful and passionate. Impetuous -- quick to anger but also the type who would get swept away in a seduction. Lots of actresses could play her. The only limitation is that she must believably resemble the Comedian...