The obvious: characters change as a result of things happening to them.
The not-obvious: When you've gotten to the point where you know what your characters will do (their actions are consistent, even predictable), when you understand the limits they're bound within, when they're no longer surprising you with their decisions, then it's time to consider changing a character's Situation.
(This especially applies to a TV series.) As an example, let's look at Buffy: Buffy's job is to slay vampires --> Her boyfriend turns out to be a vampire --> Buffy dies, killing a vampire --> Buffy is emotionally disconnected from the world --> Her boyfriend becomes an evil vampire --> Buffy kills her boyfriend, quits & runs away. ... Whenever we're just getting a handle on Buffy and the show, Whedon and his team change something fundamental, something that strikes at Buffy's core (Faith, graduating from high school, what happens to Joyce).