https://www.cinemablend.com/television/nobody-wanted-to-play-this-alien-from-outer-space-how-robin-williams-absolutely-crushed-his-mork-and-mindy-audition-according-to-henry-winkler

"...When he opened his mouth, it was like a volcano went off on stage 19 at Paramount. And I knew, ‘Get out of his way, know my lines, and don’t try to go toe to toe with this genius who was now standing in front of me.’"

I had read once the actor who was supposed to play the alien got really sick which is why they started looking again for another actor. Henry is right in saying if another actor got the role, its possible the spin off would not have happened or worked. Robin was the reason Mork and Mindy worked. His antics really could not have been duplicated and he took the role to a new level.