God he was bad in the opener for “Body of Proof”.
Did he also play Tom Packer on “The Office”, or was that just a similar-looking guy?
-Joe
Packer was played by David Koechner.
My nominee would be Garret Dillahunt - and you could show roles from the SAME television series (“Deadwood”) that couldn’t be more different.Early in the 1st season he played Jack McCall and then in the second and third seasons he played Francis Wolcott.
Don’t forget him in Primal Fear, his first role - he plays a character with a split personality, and so he switches from passive to aggressive sometimes even within one scene - its very cool and got him noticed immediately.
he’s actually playing a guy who’s pretending to have a split personality, but we don’t learn that till the end
Agree with Ben Kingsley.
And what characters could be more different than Jacob Marley, Adolph Hitler, and Obi Wan Kenobi? Alec Guinness is a must for this.
Paul Muni was famous for this (though he did go over the top sometimes). Check him out in I Am a Fugitive from a Chain Gang, Juarez, Hudson’s Bay, Scarface, The World Changes, Black Fury, The Last Angry Man, Inherit the Wind–you can scarcely believe it is the same person.
Hugh Laurie as House is so different from his British song-and-dance man that some people didn’t realize it was him.
I came in to nominate these roles. It’s an amazing example of a man being almost unrecognizable in the two roles. I had to point out to a friend that it was the same actor when we watched the series together. Although, the language in the show might be an issue for your students. Maybe you can find some clips without swearing?
James Cromwell as Fatmer Hoggett in Babe, vs. Chief Dudkey Smith in L.A. Confidential. Though the latter may be too much for middle schoolers. Probably his role in The Queen also, but I haven’t seen it. 
Try the following juxtapisitions:
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Michael Madsen torturing a policeman in*** Reservoir Dogs***, followed by Michael Madsen as the nice guy Dad in*** Free Willy***.
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Brian Cox as mass murderer Hannibal Lecter in Manhunter, followed by Brian Cox as the sweet old Scotsman telling a cute story about the pet sea mosnter he raised as a child, in The Water Horse.
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Along the same lines, try Anthony Hopkins delivering a Christian sermon in ***Shadlowlands ***(he played Narnia creator C.S. Lewis), followed by Hopkins biting off that cops’ face in The Silence of the Lambs.
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Kevin Costener ranting about the conspiracy to kill the Presdient in JFK, followed by Costner saying “I believe Lee Harvey Oswald acted alone” to Susan Sarandon, in Bull Durham.
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Gregory Peck as heroic lawyer Atticus Finch in*** To Kill a Mockingbird***, followed by Gregory Peck as a homicidal Nazi in The Boys From Brazil.
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Philip Seymour Hoffman as flamboyantly gay novelist Truman Capote, followed by a clip of Hoffman as gruff baseball manager Art Howe in Moneyball.
Edmund Gwenn in Miracle on 34th Street versus his Cockney thug in Hitchcock’s Foreign Correspondent.
Gary Cole in Office Space vs. Gary Cole in Talladega Nights
Yes, Depp has always seemed to disappear into his roles.
I was going to mention Anthony Hopkins, but I see astorian beat me to it—Hannibal Lecter, C. S. Lewis, Richard Nixon…
Henry Fonda in *Once Upon a Time in the West *vs. any other part he had.
Oh, how about cartoon voices?
Clancy Brown plays psychopaths and homicidal maniacs in most movies- kids would be blown away to see that he’s the same guy who does the voice of Mr. Krabs in “SpongeBob SquarePants” cartoons.
Or find some old commercials that show Margaret Hamilton as sweet old Cora serving Maxwell House coffee to her friends… and then show her as the Wicked Witch of the West in The Wizard of Oz?
Heath Ledger as the Joker, likewise.
What about Robby Coltrane as lovable giant Hagrid in one of the ***Harry Potter ***movies, followed by a clip of Coltrane as a hard-drinking, chain-smoking police interrogator in the British TV series Cracker?
How about Jack Nicholson is one of his many maniacal roles (MacMurphy, the Joker, whoever) followed by his performance as a sort of pathetic old man in About Schmidt?
How about Hunter S Thompson in Fear and Loathing in Las Vegas?
Oh, man… how could I forget:
Jeff Bridges as both The Dude in the ***Big Lebowski ***and as Rooster Cogburn in True Grit? A comical stoner and a Western lawman!
Blockbusters, you say? OK, how about Christian Bale as Bruce Wayne in The Dark Knight vs. Christian Bale as a washed-up boxer in The Fighter, or Christian Bale as a Cockney magician in The Prestige?