In the intro, it shows the actors who starred in the show. When Robert Culp's name is shown, they have "as Bill Maxwell" right underneath and in a box.
I presume show. He was a big star when the show started, from his own I Spy, from movies and he did high profile guest shots on shows like Columbo and Outer Limits. (His Columbo episodes are awesome.)
Did I read somewhere that he had refused, many times, to do another series before GAH came along?
Yes, that's true. He mentioned that in a few interviews I've seen with him. But he changed his mind when GAH came along because he really liked the character/script and decided to give TV another shot.
And I agree 110% about him on Columbo. The ones with Culp are easily some of my favorite episodes.
The main reason was at the time of the show's start - as Doom mentioned, Culp was a really big name and using the character's title would draw more attention to the show besides just Ralph having flying issues.
That said - in seasons 2 and three Connie's Title card had "Connie Sellecca as Pamela Davidson (then Hinkley)." Katt is the only person who did not have a title card to my knowledge, which may show that as Ralph he had a big enough attraction as was that it was not needed to know who was whom.
Back to Columbo... wow... we definitely have the same taste there. Love the Culp episodes, but the Patrick McGoohan ones are my favorites. To me, he played the best Columbo 'bad guy', with an honorable mention to Leonard Nimoy, the only person in the history of the show to make Columbo truly 'lose it' temper-wise ('A Stitch in Crime').