Comedy has been defined as “ordinary people in extraordinary situations, or extraordinary people in ordinary situations.” But what if the piece you’re critiquing isn’t comedy—or isn’t meant to be comedy? When a character you’ve come to know suddenly acts in a way that makes you stop, scratch your head, and say “huh?”, maybe there’s a problem.
Maybe. That’s an important word. What does the story’s context tell you about this new behavior? If Alice suddenly starts screaming, which she’s never done before, but it’s because the car she’s riding in just went off a cliff, that’s reasonable. If, on the other hand, as the car begins its graceful, 1,000 foot descent, she calmly takes out her .357 Magnum and blows away the driver, well, maybe not so much. OK, I can understand her being pissed at him, but really, what good does shooting him do? [Oh, right. Next, she unbuckles her seat belt, pulls her parachute from the back seat, wrestles it on—how convenient that all the straps are already adjusted to her size—opens the door, steps out, and— SPLAT! Oops, too late! Dang. Well, BASE jumping  was never her forte, anyway.]
There are lots of stories in which someone’s personality changes suddenly and dramatically, but the good ones (and some of the not-so-good ones) justify the change. You know why. And if the change makes sense, all’s well.
It is possible for an author to have a character’s behavior change significantly, or have some new and very different-from-the-norm trait appear, without explaining it or setting the context for it before or at the moment of change, but if he does, he’s asking the reader to trust that all will be made clear in good time. That’s fine, so long as he keeps that implicit promise.
Sometimes it can be hard to tell if the promise is kept, however. If you’re reading a long piece like a novella or full-length novel, but you’re only seeing it in increments (chapters), the explanation for the new behavior may not show up in the material you’re reading. In that case, you need to ask about it. If it’s a mistake, it’s possible the author didn’t realize she’d made it.
Your task as a reviewer, then, is to do a kind of sanity check on the sudden change or new behavior. Here are a few questions to ask:
- Has the author prepared me for this character’s new behavior by establishing the circumstances that warrant it?
- Has the author hinted that there might be more going on with this character than meets the eye?
- Have the character’s circumstances changed suddenly, requiring a behavior she hasn’t demonstrated before?
If the answer to any or all of these questions is “no,” then you’ll want to bookmark the moment the new behavior appears and watch for the explanation to be revealed. If it isn’t, this is something you’ll want to flag for the author.
What else do you look for when watching for out-of-character behavior?
 BASE jumping is an extreme form of sport parachuting. BASE stands for Buildings, Aerials (or Antennas but definitely NOT Automobiles, falling or otherwise), Spans (e.g. bridges), and Earth (e.g. cliffs).