I know I post too much about "WKRP in Cincinnati," but this bit of casting trivia is too good not to post and I haven't seen it mentioned elsewhere:
When casting Andy Travis, the show's lead role (or at any rate what was expected to be the lead role when they made the pilot), creator Hugh Wilson wanted to make an offer to David Letterman. The reason Letterman didn't get the part was that MTM, which was producing the show, decided that they wanted Letterman for their big production of that season: "Mary", the comedy/variety show starring MTM namesake Mary Tyler Moore.
Obviously if the part had gone to Letterman, Andy would have been more clearly the star of the show, and the show would have developed quite a bit differently than it did. (Wilson actually looks a lot like Letterman, so the character would sort have been a surrogate for the creator.) The same goes for the original choice for Mr. Carlson (according to director Jay Sandrich), Roddy McDowall.
Incidentally, the casting idea Wilson wanted to pursue -- casting a not-particularly-famous comedian who was clearly headed for big things -- was a strategy that other sitcoms used in this period, casting a young comedian in the "straight man" role. The creators of "Taxi" used the young Martin Short as the lead character of the excellent law-office sitcom "The Associates," and Allan Burns (co-creator of "Mary Tyler Moore") used the young Jim Carrey as the straight man on "The Duck Factory."