Supposedly he was offered fhe role of McCoy but turned down
He was offered the doctor's role in "The Cage." He said no because he believed that the first or second leads in a TV show had a future, but if you play the third lead or lower, all your future job offers will be down at that level.
To support this theory, Throne cited Paul Fix (Dr Piper in WNM), who wrote a book called "Third Man Through the Door" about spending his whole career in third-lead parts. But Fix was never going to be a star regardless. And look at Lee Majors. He started in supporting parts and climbed the ladder to play first lead in THE SIX MILLION DOLLAR MAN and THE FALL GUY. So I think Throne's theory was completely bogus. You've either got star quality or you don't.
Anyway, Throne loved the script for "The Cage." He wanted to play Spock but Nimoy already had it (thank God). So he offered to voice the Keeper. It's in his STARLOG interview (Issue 190, May 1993).