B.J. Novak (Ryan) isn’t officially leaving The Office, but he’s taking a “leave of absence” while he does a Quentin Tarantino movie, and it’s entirely possible that he might not come back at all. (Novak’s character isn’t all that essential any more, but he’s one of the best writers on the show.)
Ryan, whose presence in the opening credits is probably inexplicable to anyone who started watching the show after season 2, is an example of what I’ve previously called “Potsie Syndrome,” a character who is supposed to be important, but remains on the show even after his job is over. (Ryan was supposed to carry at lest some of the burden of being the “sane” one in the office; unlike Jim, who maintains his sanity by being a slacker and not really caring about his job — or anything except Pam — Ryan was the one with ambition, scheming to get out of the office and into something better. But the end of season 3 was the end of that storyline, and what does he have to do when that’s finished? Except for drugs, not much.) He’s also an example of a character who is downgraded from appearing in every episode to being a semi-regular, although I won’t come up with a cutesy syndrome name for that.
This type of character is one who appears in the opening credits, is billed as a regular and (presumably) paid as one, but doesn’t actually appear every week. Sometimes this is written into the actor’s contract, like Andy Kaufman on Taxi. (He had it in his contract that he only had to appear in about half the episodes, and although — contrary to legend — he grew to like the show and agreed to appear more often, he never appeared in all the episodes in any one season.) Sometimes it’s just that the actor is allowed to do something else rather than stick around for an episode where he/she isn’t really needed. I may be wrong, but I’ve always assumed that that’s why Jane Krakowski sometimes misses episodes of 30 Rock; they could give her one token line, but they don’t. And then you get the Ryan type of situation, where someone is actually less of a regular than some of the actors who aren’t in the opening credits. Catalina on My Name Is Earl also fits into this category. And on the new 90210, Jessica Walter was hired as a regular and downgraded after the producers realized that she had like two lines per episode.
Obviously there’s a lot of overlap here with Potsie Syndrome: every downgraded regular suffers from Potsie Syndrome, but some Potsie Syndrome sufferers will continue to appear in every episode, if only for one line a week.
Comments section is open for the names of characters who started as regulars and then — without an official announcement — started appearing only in selected episodes.