By this I mean a TV actor or actress who played a role, (meaning in a series not as a one time shot), where they played totally opposite of their previous role
For instance, Larry Hagman, played Major Nelson on I Dream Of Jeannie. He was a great guy and very nice. Then he goes on to play JR Ewing, on Dallas, one of the most evil and hated people in TV history.
Another example is Conrad Bain, he played Arthur Harmon on Maude. He was very conservative and old fashioned. Then he switches over to Diff’rent Strokes and Mr Drummond was very liberal and forward thinking.
Both Rue McClanahan and Betty White, played opposites of their previous roles on the Golden Girls
You see what I mean by playing roles at the opposite end.
I mean regular TV series with regularly appearing cast members, not one or two time shots, cause it’s always easy to find actors who do that.
I mean Bob Newhart basically played the same guy on both his TV series.
Carrol O’Connor. He played Archie Bunker, a conservative bigot, on All in the Family and then Chief Bill Gillespie, a racially progressive liberal, on In the Heat of the Night.
Then-marrieds Barbara Bain and Martin Landau in their divergent parts in Mission: Impossible versus their sci-fi Space 1999 show. (Heck, add Leonard Nimoy to the Star Trek vs. Mission: Impossible** stint.)
Four actually: The Bob Newhart Show, Newhart, the rather short-lived Bob and the equally short-lived George & Leo (which starred Judd Hirsch after Indenpendance Day, so perhaps he was the headliner).
Taking the OP overly literally, Victor Garber from ALIAS is a TV actor who played a role – Jack Bristow, the brutal secret agent who sleeps with hot chicks and excels at torture – that was the total opposite of a previous role, when he’d earnestly played Jesus as a twentysomething song-and-dance man in clown shoes who goes around cheerfully face-painting flower children and playing jaunty piano tunes in GODSPELL.
Ben Kingsley won an oscar for his portrayal of Gandhi, and was nominated for another for his portrayal of psychopath’s psychopath Don Logan in Sexy Beast. And yes, I’m aware those are films and not TV shows, but it’s still the canonical example for me.
Patrick McKenna who was excellent at playing Marty Stephens (the hot-tempered, cold-hearted, head floor-trader of the frictional bank of Gardner-Ross) in traders, while concurrently playing Harold Green (the geeky nephew of the title character) on the Red Green Show.
It was literally: wake up; eat breakfast; star in a comedy in the afternoon; star in a dramatic role in the evening…