Quincy ME

Marc Scott Taylor was originally hired as a technical advisor but became a semi-regular cast member because he could operate electron microscopes and other complex instruments. It was more cost effective to give him a recurring bit-part than to train the actors to operate the equipment convincingly. His role was greatly expanded in an episode in which "Sam" had been poisoned and "Mark" helped Dr. Quincy save his life. He eventually provided a couple of scripts for the show and by season seven had become a co-producer.