I’d agree that Williams used to always play “Wacky unconventional X who nevertheless gets the job done”, but he seems to have been diversifying with his movies a bit recently. Some, like Death to Smoochy are obviously extensions of parts of his stand-up persona but (although admittedly I haven’t seen either) his performances in Insomnia and One Hour Photo are by all accounts ‘proper acting’.
The Murray of Stripes and Ghostbusters was a wisecracker par excellence and had oodles of Irish gumption. But he was neither cocky nor arrogant. In fact he was often up against icons of traditional, obnoxious male arrogance – Warren Oates in Stripes, William Atherton in Ghostbusters – and overcoming them with sheer wits and wit.
Joahn Cusack (Joan and John)
James Spader
Although I pause here to point out that in the movie True Colors, John Cusack and Spader each play roles you would envision better suited for the other.
Tim Allen.
Owen Wilson always plays himself, but I like him and find him very enjoyable to watch.
OK, maybe she’s done other stuff, but in every movie in which I’ve seen Winona Ryder, she’s been this fragile, doe-eyed, soft-spoken, insecure little thing. Anyone care to comment?
No mention of Meg Ryan?
Jeesh. I can’t go and see a movie with her in it at this point (the last one, something about time travel–Kate and Leopold–was just soooo bad).
Nicole Kidman–ok, I’ll give you the Hours, but what else? Nadda.
Winona played a seemingly soft, fragile, innocent young thing in Scorcese’s --gah! Cannot remember the title! It had Daniel Day Lewis in it, based on an Edith Wharton novel–anyhoo-she turns out to be a scheming, controlling puller of strings.
I think the OP means something that goes beyond typecasting. Like in “You’ve Got Mail”–Meg played Meg and Tom Hanks phoned his performance in.
I don’t see Hugh Grant doing this so much anymore. “About a Boy” was a much different character–not as nice, not as “heroic”, not as “golly, gee whiz” as others.
And he played a total shit in the first Bridget Jones.
Crikey, how about Jean-Claude Van Damme?
Brian Dennehy
Chris Tucker
Christopher Walken
Here, here. I was shocked at how well he did in Matchstick Men.
I nominate Martin Lawrence.
Ooh have you seen The Spanish Prisoner? He’s definitely not doing the “funny guy” thing in that movie, which I found very entertaining.
I’ll second Owen Wilson, he can only play “himself”.
Did anybody mention Bill Paxton? And he is a bad actor in everything I’ve seen him in.
The Age of Innocence?
Why would it necessarily be bad acting to play yourself? Wouldn’t good “method” actors actually seek this out by first changing their own personae into those of their characters, and then playing themselves?
Jerry Lewis as Syd Valentine in The Big Mouth comes to mind - I seem to misremember a TV special on Jerry Lewis (maybe it was E! True Hollywood Stories) claimed that he was a terrible person to be around even when “off duty” because he was “in” the Syd Valentine persona.
Perusing the movies of Jerry Lewis, I am given to wonder how well he played his seven distinct characters in The Family Jewels.
Either Jerry Lewis is the best example or the worst example of an actor playing himself. Maybe both.
Samuel L. Jackson has been playing Jules his entire career. It doesn’t help that he always screams his lines.
Tommy Lee Jones. Usually the defining characteristic of each of his characters is what the character wears. Thus: In FireBirds, he plays Tommy Lee Jones in a flight suit, Men In Black, he plays Tommy Lee Jones in a suit and sunglasses, and in Man of the House, he plays Tommy Lee Jones with a cowboy hat and a cool-looking Colt .45.
That said, I LIKE Tommy Lee Jones. He’s a very entertaining character. 
Clive Owen seems to play the same sort of cool (attitude wise) British guy in everything I’ve seen him in (Which includes Sin City, The Driver, and Privateer 2: The Darkening)
Oh, and as for Arnold Schwarzenegger, in “Last Action Hero” he briefly played Arnold Schwarzenegger. 
One of my favorite things is when an actor plays a character who lampoons the characters he normally plays, ie: Antonio Banderaz in Spy Kids (featuring the entire cast and the director of Desperado) or Shrek. 
Charlie Sheen has also done this from time to time (such as in Hot Shots Part Deux, where he even goes so far as to repeat his monologue from Platoon, while Martin Sheen passes on another riverboat repeating HIS monologue from Apocalypse now)
Mark Hamil also does this. 99% of the stuff I’ve seen him in (ie: Not voice acting) has him playing Mark Hamil. (And it makes me grin every time)
I disagree, particularly in Unbreakable. Even if you hated the movie (which I didn’t by any stretch), I thought he played a very different role than his usual. Same with Sphere, which I did hate. He was terrible in it, but it was different.
But I think you’re right for about 90% of his films.
Yeah. He’s lucky- he has a solid, respected career as a voice actor, and he’ll probably always have cameo requests because he was Luke Skywalker.
Ahemm!! Valley Girl?? C’mon man, he was the Punk Dude who totally stopped the world and melted with Julie!
I liked Leaving Los Vegas, but right about Face Off was when Cage became a complete tool.
Heh, in some of the Wing Commander games (3, 4, and Prophecy) he plays a war-weary starfighter pilot. About a third of the blooper reel for WC3 is actors forgetting their lines and then pointing off screen and asking “Isnt’ that the guy from Star Wars?” Best Mark Hamil cameo ever though has to be the one in Jay and Silent Bob Strike Back.