I was lucky enough to be able to attend a taping of Mork & Mindy after Winters had joined the cast, and I can confirm he and Williams did more improvisation off-camera than on-camera. In fact, the taping I saw ran almost two hours longer than usual because they could barely get Winters and Williams to stop clowning around long enough to actually film the show, and they had to re-shoot a few scenes because Winters and Williams were improvising and went so far off-script that they couldn't use the footage for the episode. R.I.P. Mr. Winters, I'll miss you.