
If you watch television long enough, you begin to realize that there are different acting tracks. For example, actors on mystery shows continually do mystery shows; actors on sci-fi shows continually do sci-fi shows. This is probably partly due to recommendations between casting directors and partly due to the "resume" that an actor builds up over the years.
Sci-fi and fantasy are far more respectable now than they used to be, but for years, every actor in Hollywood wanted to break into "real" television or movies, i.e. drama. Hence, Shatner and Nimoy's desperate attempt, after
Star Trek: The Original Series, to lose the "captain" and the ears. (Nimoy did a far better job
gracefully accepting the inevitable; Shatner has sort of come around.)
It would not be until Jonathan Frakes that an actor would frankly admit, "This is exactly the career I want." (DeForest Kelley felt the same way--he just wanted the work--he just never said so very loudly.)
However, despite its rise in respectability, sci-fi and fantasy still have a stigma--as they do in higher education. The days of trying to separate genre sci-fi from "literary" sci-fi are beginning to fade, but the issues are still there. Patrick Stewart was able to (partly) break free of the
Star Trek "stigma" because (1) he'd had a fairly strong pre-
Star Trek career; (2) his fans were willing to let him go (in fact, Trekkies are rather proud of Stewart's Shakespearean proclivities); (3) he's British, and British television is a smaller world that allows for greater flexibility, not because it is inherently flexible but simply because Dr. Who actors need to be able to do Agatha Christie and costume dramas (even Hugh Laurie could probably go back to doing Austen if he really wanted).
Still, I have always held a special place in my heart for those non-sci-fi track actors in Hollywood who show up on sci-fi shows. Granted, sometimes they just need the work, but sometimes, they really do love sci-fi and voluntarily make appearances.
Kelsey Grammer: Star Trek: Next Generation, "Cause and Effect." He took the part of Captain Bateson while he was doing
Cheers, a year before
Frasier. To continue the fun,
Frasier episodes include a boatload of
Star Trek references. In general, the Trekkie on
Fraiser (Noel) is the butt of those jokes, but the sheer number of jokes reflects a profound knowledge of the franchise.
Wayne Brady:
Stargate SG-1, "It's Good to Be King." I get a kick out of Wayne Brady's willingness to do just about anything from
Whose Line Is It Anyway to
Robot Chicken to
How I Met Your Mother. He really does seem to have fun no matter what. He doesn't even sing on
SG-1; he just plays a bad guy!
Whoopi Goldberg:
Star Trek: Next Generation. The story of how Whoopi Goldberg told LeVar Burton she wanted to be on
Trek but he didn't think she was serious until she kept insisting has been told elsewhere (and better). It is still impressive that she appeared so consistently on the show.
Star Trek: Next Gen, despite various faults, has remarkable casting (and is the
Star Trek that I've invested in the most).
Speaking of
Star Trek, two actors with wonderful voices (and long television careers) who show up throughout the franchise are
James Sloyan and
Alan Scarfe. But then, they've done everything.
Another wonderful sci-fi guest star is
Saul Rubinek. He may be better known, now, for his work on
Leverage, but he did appear in one of
Next Generation's classic episodes "The Most Toys." He also does a great job in
SG-1's "Heroes."
I rather adore
Alan Rachins (Dharma's dad on
Dharma & Greg). He's another working actor. He's appeared in a wide range of material from
L.A. Law to
Lois & Clark to
SG-1. He wants to work, so he works. He doesn't turn up his nose at doing a voice for the
Spectacular Spider-Man!
James Cromwell could have stuck to the drama circuit if he'd like but he did several
Star Trek: Next Generation episodes in the early 1990s and from that point on, more and more sci-fi shows appear on his C.V. (and of course, he IS Cochran!).

Ending where I started, another
Cheers/Frasier alumnus,
Bebe Neuwirth makes a guest appearance on
Star Trek: Next Generation in the very funny episode "First Contact." No, she doesn't play the scientist. She plays the utterly obsessive "fan" (who wants to sleep with an alien: Riker). The role is hilarious because Neuwirth plays it straight. And she doesn't act anything like Dr. Sternin!