Make your characters act in a way that keeps things happening, and seems reasonable to the reader. All characters do things based on anger, fear, happiness, disgust, shame/guilt, sadness, desire, pity, love, discovery, and other emotions. In other words, they act based on how they feel.
This has a bit to do with characterization, which I admit I'm not the strongest with, given my history with fan fiction (where you don't need to characterize). I don't see this as being a big problem in my works or many works I read. It has to do with A) characters acting the way the reader thinks they're gonna act and B) Having the story unfurl fluidly. With original characters, it's hard not to know if they're acting the way they should or shouldn't, especially in short stories. What most people screw up is whether ANYONE would act/react that way. Then you have a character acting "out of character", a phrase a dreadly hate, coming from a fan fiction background. Comes up a lot especially in horror and sci-fi, and it's hard because no one really knows how they'd react to an Atomic Space Bride Monster unless they saw one. That's why you gotta make it up, but make it up plausibly. So that's part A. Part B is related to the story/plot. Anyone, ever, does the things they do based on one buddhic principle - desire. Everything else is just details. When you hit a roadblock in a story, or something like that, get a hold of what does your character desire. What do they want? What do they need to do to get what they want? So you have a given situation, and thusly, in this given situation the character must feel something. Based on that feeling, make the character act. Although, my biggest problem is making up a good situation to get the character feeling something, and then getting him to act.