William Daniels=John Adams
Marion Ross=Marion Cunningham
Bob Newhart=Bob Hartly/Dick Louden(who were, in fact, the same guy. . .)
For me, Daniels will always be K.I.T.T.
Also, St. Elsewhere’s Dr. Mark Craig.
I doubt that I’ll ever see or hear William Daniels without thinking of Captain Nice.
Shatner will always be Kirk and Nimoy will always be Spock. Same for everyone else on that show. “Hey, it’s Scotty!” “Hey, it’s Uhura!” “Hey, it’s Bones!”
David Boreanaz will always be Angel. I keep waiting for him to bite Tempe on the neck. Hey, maybe that’s why they haven’t had sex!
To me, Tommy Lee Jones will always be Dr. Mark Toland of One Life to Live.
“He said he was going to let me fire the proton torpedoes at the Nazis.”
Vigo Mortensen will always be Satan to me. It makes LOTR soooo much more interesting.
Keanu Reeves will always be Ted Logan from Bill and Ted’s Excellent Adventure.
In fact, I have retitled his movies to Ted Goes Surfing, Ted Goes to Court, Ted Hacks Some Computer Networks, Ted Drives a Bus Really Fast, Ted Plays Football, Ted Goes to Hell, etc.
Darren McGavern will always be “the old man” from A Christmas Story.
Zero Mostel will always be Max Bialistock.
Meg Ryan will always be Sally.
seconded (I was raised on the original 1776 soundtrack)
Just look at his IMDb profile, he’s played John, Sam, or John Quincy so many times he might as well change his name.
…shoulda gotten a Tony for it, too.
“Is Anybody There?”
"Does anybody care. . .
Does anybody see. . . what I see?"
Dominic West is Jimmy McNulty. Period.
Now that is why I can’t get interested in any of the other Tom Clancy movies. It’s the wrong actor. 
They just seem off, to me.
That made me laugh. 
Ray Winstone will always be the terrifying abusive bastard from “Nil by Mouth” to me. Watching “Sexy Beast,” I was on the edge of my seat, waiting for old Gal to start wailing on his wife, pool boy, friends, Ben Kingsley, the patio furniture, the palm trees, etc.
Adam Baldwin will always by “My Bodyguard” to me. Even when I saw him in “Full Metal Jacket”, I said “it’s My Bodyguard !”
Willem Dafoe (and I love his work) will always be Raven from “Streets of Fire” to me.
Peter Falk will always be Columbo. Always.
And of course, I don’t think it was a coincidence that Apollo 13 had all the trouble it had with Forrest Gump driving it.
+1
Sigourney Weaver will always be Ellen Ripley, and perhaps by association, Ian Holm will always be Ash because I seem to have some nervous expectation that he’ll start sweating and/or vomiting milk every time I see him.
Hey, just because that’s how most people react upon meeting you doesn’t mean Holm will do the same.
Who said anything about meeting anyone?