When you start up with a story, you often begin with a certain character in mind. However, other times, you focus on the plot first, then put in characters. For me, it is often difficult to come to grips with a character at that point, since he will be simply another actor. It takes time to develop a personality from nothing, so it’s especially problematic in short stories.
To combat this, I like to put my characters into personality archetypes and work from there in order to build up to more solid characters. Otherwise they simply drift along with the story, never really sticking.
As a base, I sometimes just come up with something that fits the role (i.e. a rebellious teenager in a horror story), and other times I use one of the archetypes of several personality tests, like the Myers-Briggs.
Archetypes are usually bad because they put your character into a box and leave him there and then you have a character who is not developing, and a cliché. So the important part of picking an archetype is to build from it, not just lean on it for support. It’s a lot like structure in stories.
If you can make a character without archetypes, do so immediately and joyfully. If, however, you struggle to get a solid mass from your collection of words, consider picking a base, a structure, to help you build them up.
Using archetypes also allows you to deviate easily (since you know exactly who your character is—at least in the beginning) and therefore surprise readers convincingly.
Like adverbs, archetypes should be avoided if possible (except perhaps with minor characters that have two lines of dialogue), but used if necessary. It is especially useful when there are multiple characters in order to differentiate between them and make them memorable, as Orson Scott Card did in Speaker for the Dead.