William Daniels as Dr. Mark Craig and as K.I.T.T.
Several bad guys on Oz played good guys on Law & Order.
Chris Meloni: Chris Keller on Oz vs. Elliot Stabler on L&O:SVU
Dean Winters: Ryan O’Reilly on Oz vs. Brian Cassidy on L&O:SVU
Stephen Colbert played a twisted, clever forger and murderer on an episode of Law & Order: Criminal Intent. He was actually really good at it.
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My personal theory is that Modern Family is the spiritual sequel to MWC. Remember that the Bundy curse was lifted when Al defeated Igor in a jousting contest in Lower Uncton. The problem was that Peg was part of the curse, and she remained afterwards. I believe that after Peg died, Al was able to use his formidable talents to start a thriving shoe business, as well as finally succeed with one of the gorgeous women he was always meeting. Kelly got hit on the head again, and regained her intelligence (note that they often refer to Claire’s colorful past) and Bud finally gave up on women and embraced his true nature.
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Claire and Mitchell’s mom is still alive (played by Shelley Long), and her character is nothing like Peg Bundy.
Kathryn Erbe: Death row serial killer Shirley Bellinger on Oz vs. Detective Eames on L&O: CI
That episode sucked anyway! Besides, that’s why I say “spiritual” sequel. If it wasn’t for that POS episode I’d claim it was the actual sequel. Notice how Mitchell is shorter than Claire?
Wow, I don’t see that at all. They both seem to be socially awkward, semi-creepy characters to me; it’s just that the situation around them is different (i.e. funeral home vs. office). I’d love to see him tackle an entirely different kind of role one day, but I don’t think this is it.
Stephen Colbert has 28 acting credits on IMDB.com, none of them as conservative pundit “Stephen Colbert,” much to my surprise.
Kerri Kenney played a pretty brash Euro-babe on Viva Variety. She played a mousey, shy, socially unaware cop on Reno 911.
Me, too! I even have it on VHS. I’m such a nerd.
Aaaand now the smarmy gardening/pool cleaning father on “Raising Hope.”
…and Root appeared on “True Blood” as a gay will-let-you-bleed-me-for sex milquetoast character with quite an, er, explosive end.
He also played “Eli, the stand in for Jesus” on Xena, which is wildly different than Lassiter (I missed Jericho)
Peter Davison went from the undependable but charming Tristan Farnon in “All Creatures Great and Small” to a real bastard plastic surgeon in “Distant Shores”. (Between those he also played Doctor Who, but since I’m not a fan, I don’t know where that fits in.)
Ian McShane of Deadwood played a handsome, urbane, young European tour guide who romanced Suzanne Pleshette in the movie “If It’s Is Tuesday, This Must Be Belgium”.
And Rock Hudson, famous for his roles in romantic comedies with Doris Day, played a seductive high-school football coach/serial killer who had affairs with some of the school’s most beautiful students and then began murdering them for various reasons in the black comedy “Pretty Maids All In A Row”. Then he did another 180 and began playing Commissioner Stuart McMillan in one of my favorite husband-and-wife crime series, McMillan & Wife.
How about Gary Cole, from good guy hero in Midnight Caller to sinister, quite literally devilish Sheriff Lucas Buck on American Gothic?
This happens often on soap operas, where actors often move from playing nice guys to malicious bastards, sometimes on the same show – sometimes simultaneously. (Evil twins and multiple personalities, don’tcha know.)
One very acclaimed example: Ellen Wheeler, Anne Heche, and Jensen Buchanan each played twins Marley Love (sweet, repressed, ethical) and Vicky Hudson (spicy, sharp-tongued, slippery-moralled) on Another World. Unlike many “evil twins,” Vicky wasn’t just brought on to be a psycho villain and wind up in jail; both roles were longstanding characters and appeared on the show simultaneously for years. Wheeler and Heche both won Emmys for the roles in different years; Buchanan was nominated but didn’t win.
On Days of Our Lives, Judi Evans Luciano played kind-hearted, down-to-earth Adrienne Johnson for many years during the '80s-'90s; later she was inexplicably brought back to play trashy, greedy, manipulative Bonnie Lockhart. (I say ‘inexplicably’ not because she was bad in the role – she wasn’t – but because members of Adrienne’s family were still on the canvas, so it was ridiculous to have the actress playing a different role opposite the same people who had once played her other character’s brother/sister-in-law. But soaps are notorious for insulting b.s. like that.)
Then you have the soap hoppers. Matthew Ashford played bland good guy Drew Ralston on One Life to Live, moved to down-to-earth, working class supportive hubby Cagney McCleary on Search for Tomorrow, then had his 180 degree turnaround by playing sophisticated, educated, scheming, wealthy wife-raping-but-hating-himself-for-it villain-turned-antihero Jack Deveraux on Days of Our Lives. Next he bounced back to playing a more traditional white bread hero Tom Hardy on General Hospital. Then another flip-flop when he returned to One Life to Live, this time as a brilliant professor haunting his campus as a sadistic serial killer.
Oh, and speaking of One Life to Live, Judith Light played tormented ex-prostitute Karen Wollek before moving to primetime comedy Who’s the Boss as fairly prim, upper-class executive and divorced mom Angela.
I could go on with dozens of other examples, but no one wants me to. 
“My daddy’s havin’ a heart attack!!!”
Heck, Steven Root in “News Radio” vs. everything else he ever played, from “Office Space” to “Justified,” and everything before, after and in between.
He pretty much fills the shoes of M. Emmit Walsh as the consummate character actor.
William Frawley played super grouch Fred Mertz on “I Love Lucy” and later was the somewhat kinder father-in-law substituting as housewife on “My Three Sons”. Of course Fred MacMurray had played heels in movies like “Double Idemnity”, “The Caine Mutiny” and “The Apartment” before super dad Steve Douglas.
Never saw the tv series of “Nine to Five” but didn’t Jean Marsh played the brown nosing snitch after playing Rose in “Upstairs, Downstairs”?
No to mention the badass terminator Cromartie, and the infant-like proto A.I. John Henry, both on Terminator: The Sarah Connor Chronicles. Do not attempt to catch up on the last few episodes of T:TSCC interspersed with watching Raising Hope. Damn near gave me whiplash. (Watch out, Maw Maw! There’s a terminator behind you!!)
I like Dillahunt, but lets be honest - playing a badass terminator does not take much in acting skills. He does have the intelligent sociopath role nailed in recent years, doing turns on the short-lived Life and in Burn Notice. I was surprised to see him doing something so different in Raising Hope, but not surprised enough to give it a second episode.