Tom Lennon, Robert Ben Garant, and Kerri Kenney were all on The State, and then Reno 911.
Ade Edmondson and Rik Mayall:
Vyvyan and Rik in The Young Ones
Various characters in Comic Strip Presents
Eddie Catflap and Richie Rich in Filthy, Rich and Catflap
Eddie and Richie (different!) in Bottom
Also, Ade Edmondson, Rik Mayall and Nigel Planer:
Vyvyan, Rik and Neil in The Young Ones
Various Character in Comic Strip Presents
Eddie Catflap, Richie Rich and Ralph Filthy in Filthy, Rich and Catflap
David Mitchell & Robert Webb:
Bruiser
That Mitchell & Webb Situation
That Mitchell & Webb Look
Peep Show
:smack:
I got nothing.
It seems like there’s a LOT of crossover betwen Oz and the many Law and Order programs. J.K. Simmons was a badass neo-Nazi convict on Oz, but a nice psychiatrist on ***Law and Order. *** Meanwhile Kathryn Erbe played a murderous Mom (inspired by Susan Smith) on Oz, and a detective on Law and Order: Criminal Intent.
Buffy the Vampire Slayer ran from 1997-2003, won Emmys, launched a spin-off series and has generated millions in merchandising. Many writers would kill for that kind of failure.
Denis Leary, Adam Ferrara, Lenny Clarke, Diane Farr all appeared in both The Job and Rescue Me, both of which were created and written by Leary and Peter Tolan.
Martin Landau and Barbara Bain were both on Mission: Impossible and Space: 1999 (and married to each other as well).
Bill Bixby:
My Favorite Martian(107 episodes)
Courtship of Eddie’s Father(73 episodes)
Incredible Hulk(82 episodes)
Unbelievable. :rolleyes:
A number of cast members of “Battlestar Galactica” (notably Michael Hogan and Tahmoh Penikett) also were regulars on Canada’s “Cold Squad”.
Who were the co-stars that also were on both series?
Both y fiancee and I have not appeared on several TV shows. Hey! Maybe we both didn’t co-star with your uncle together! Wouldn’t that be weird?
Lennon and Kenney were also M’sieu and (the former) Madame Laupin on Viva Variety (a show which wasn’t great but definitely had its moments).
Andy Griffith and Don Knotts starred together on “The Andy Griffith Show”; Knotts played a recurring character in 15 episodes of Griffith’s “Matlock”.
Steve Carell and Ed Helms both appeared on The Daily Show and The Office
As true now as it was in post #2, and yet still not what was asked for in post #1.
My cat’s breath smells like cat food.
Actually the first trio was Winters, Meloni and Wong from Oz and Law & Order: SVU. 
There’s also a fair bit of crossover between the Law & Order universe and the Homicide universe but not so much in regular cast members, save for Richard Belzer.
Belzer plays the same character in those shows, though. The OP asks for the same actors playing different characters.
Yeah, I suppose Buffy has to count as successful; I always think of cult shows as not successful. But in my mind I consider Rescue Me successful, and yet it almost certainly has fewer viewers than Buffy did.
That opens up cable “hits”:
Edie Falco was Carmelo Soprano and Nurse Jackie
The guy who played the priest Carmelo was hot for on the Sopranos plays the pharmacist on Nurse Jackie.
The priest guy wasn’t really a main character, though. Hmmm, this is still tough.
Anyone else from Six feet Under on Dexter?
Half the cast of Deadwood was on John from Cincinatti, but the latter sucked balls and wouldn’t qualify for any definition of successful.
Was anyone from Arliss on Entourage?
Anyone from The Gary Shandling Show on Larry Sanders?
I got nothing.
Both Ben Brower and Claudia Black from Farscape ended up together on Stargate SG-1, though both were late additions/substitutions to the cast.
Nice win.