Armin Shimmerman was Principal Snyder from Buffy; Harry Groener played Mayor Wilkins.
As for me:
Michael Madsen = Mr. Blonde (Reservoir Dogs)
Samuel L. Jackson = Jules Winfield (Pulp Fiction)
Alan Rickman = Hans Gruber (Die Hard)
Gary Cole = Bill Lumbergh (Office Space)
James Cromwell = Lt. Dudley Smith (L.A. Confidential) – I always want to warn other characters in other Cromwell movies not to turn their backs on him!
Bruce Campbell = Ash (Evil Dead 1 & 2, Army of Darkness)
Sorry, Alan Alda, but to me, you will always be Hawkeye Pierce (MAS*H). You can pretend to be a US senator, but a Korean War doctor you’ll always be.
Grace Kelly is and will always be Lisa Carol Fremont from Hitchcock’s Rear Window.
Mr. Thorwald was played by Raymond Burr.
The first time I ever saw Anthony Anderson was in his amazing, and apparently convention-defying, role in “The Shield”. To this day, I regularly forget the poor man’s name and refer to him as Antoine Mitchell. With this in mind, it’s now hilarious to watch the many, many films in which he plays his usual loveable, bumbling goofball, and imagine Mackey and the Strike Team bursting in the door and slamming him to the ground at any given moment.
Even more amusing was watching him in one of those Celebrity Poker tournaments. When he lost, I kept waiting for him to flip the table over, calmly break Amber Tamblyn’s neck for talking smack, stroll on into the Loser’s Lounge and make Dave Foley do unspeakable things to him on national television. When Antoine goes all-in, if your name ain’t Vic, you damned well better fold, bitch.
Heh, no one has mentioned Tom Selleck yet. He will always be Dr. Richard Burke.
Just kidding, I mean Magnum, of course. 
Not to me, although he was deliciously hunky in that role.
I always see Chris Knight (Real Genius) when I see Val Kilmer.
Humphrey Bogart - Charlie Allnut (or Rick)
Orson Wells - Harry Lime
Charlton Heston - Chrystagon De la Crue
John Wayne - Rooster Cogburn
That’s my point. Raymond Burr was Perry Mason.
Aaaand, now that this zombie thread has come back to life, I may as well bring up Ian McDiarmid. It was kinda hard watching him in Elizabeth I on HBO without reimagining Lord Burghleyt’s lines in the Palpatine voice.
“The bloody and deceitful man shall not live out half his days…Master Jedi.”
Bonks self on head. D’oh!
Yep, I’m a doofus.
Same guy played both characters. Dunno why I thought differently.
Can anyone see jason alexander as anyone besides George Costanza? Or Michael Richards as anyone but Kramer?
George Constanza can be Duckman and Michael Richards can be Stanley Spadowski from UHF. Though most of the times, they’re not.