Occasionally, TV shows have moments that aren't scripted, whether they're improvised jokes or sudden dramatic inspirations, which prompt real reactions on set.
Whoopi Goldberg didn't need to do Star Trek during her career boom in the '80s. She did it because she wanted to, which Patrick Stewart didn't initially get.
Few actors could go toe to toe with Michael Dorn on Star Trek, but Suzie Plakson did so brilliantly, but that itself started a small production crisis.
A lesser-known Amy Adams appeared on both Charmed and Buffy the Vampire Slayer in the same year, and gave a predictably fantastic pair of performances.