John Cazale as Pacino’s sidekick in Dog Day Afternoon and DiNiro’s sidekick in Deer Hunter.
Pity he never did anything else. 
Stalone to me is pretty much either Rocky or Rambo.
Similarly, Carl Weathers is either Apollo Creed or whatever role he was in Predator (cameo in Arrested Development aside).
Patrick Stewart to me is mostly Jean-Luc Picard and partly professor X.
Miley Cyrus managed to fit 2 claims to fame in the same show.
Yep, I wondered whether that would qualify but unless she originated a famous SNL character that I’m not aware of I didn’t think it would count.
Bob Newhart, at first a standup comic, but best known as Bob Hartley and Dick Loudon.
Or maybe they were the same character. That’s the wonder of TV.
This is the one that came to mind immediately. I remember the first time I realized it was the same person. I’ve never seen her in anything else and don’t want to.
Another one is the girl in Revenge of the Nerds who was the Omega Mu girl that was Gilbert’s girlfriend (I think her name was Judy). She also played Jordan in Real Genius (hot smart girl who talked very fast).
Stallone is only Rocky and Rambo. He created two massive, distinct character franchises.
LeVar Burton: famous as Kunte Kinte (Sp?) in ‘Roots’ and Geordi LaForge in ‘Star Trek: TNG.’ (Also famous for hosting ‘Reading Rainbow,’ but unless I’m misremembering he was himself and not playing a role in that.)
Mary Tyler Moore, as Laura Petrie in the Dick Van Dyke Show and Mary Richards in her own show.
Correction: Limpet
I came in here to mention her: Michelle Meyrink. I loved her in Real Genius (power-sanding her dorm room floor, knitting a sweater for Mitch). Hasn’t appeared in a movie or TV show since 1988.
Okay, Gurney Halleck aside, why no love for Deputy Director Bullock?
I’ll submit Raymond Burr for Perry Mason and Ironside.
The extremely excellent documentary I Knew It Was You about his career highlights this fact. He was a weird cat, but a hell of an actor.
Similarly William Shatner is pretty much Captain Kirk or TJ Hooker. Yeah he’s had a career of roles basically mocking himself as stunt casting since then, but you’d be hard pressed to say he was famous for any of them.
Denny Crane!