Linda Hamilton
Linda Hamilton was a staple in the first two Terminator movies. Unfortunately, she couldn't bring herself to be on board with the third movie's script so she turned down the role. The writers killed her off during the timeline between T2 and T3.
Will Smith
Independence Day is probably one of Will Smith's best films. Even with that being said, the actor did not return for Independence Day: Resurgence in 2016. This was actually due to scheduling conflicts. "It was one of those things," he said. "I had a couple of films lined up, I had Concussion and Suicide Squad, and so it was a decision, timing-wise, between Independence Day and Suicide Squad." His character, albeit the main one, was killed off-screen.