Something's starting to happen to one of our characters - maybe he's been infected with some virus... Maybe he's going crazy... Maybe he's just gaining weight. Whatever it is, it's usually not directly explained, and we don't see him changing on-screen... It's just that one day, he looks a little bit... off. And each time we see him after that, the change is a little more noticeable, usually until it reaches either hilarious or horrific levels. As mentioned above, this is always played for either drama or laughs. May also overlap with Metamorphosis, which is often a gradual process.