Malcolm Gladwell writes in What the Dog Saw, "Good writing does not succeed or fail on the strength of its ability to persuade. It succeeds or fails on its ability to engage you, to think, to give you a glimpse into someone else's head." This is true whether you write non-fiction, novels, or the blend of these two: the memoir. Getting a reader into another head? That's the work of good character-building.
Building characters comes from a knowledge of behaviors. The Meyers-Briggs personality tests rank people in four areas, using questions that measure whether you are more of a:
• Introvert vs. Extrovert
• Thinking vs. Feeling
• Sensing vs. Intuitive
• Judging vs. Perceiving
Giving yourself a test lets you ally yourself closer with one of the ITSJ-EFIP combinations. It's a great starting point for understanding aspects of a character. The book Plot vs. Character outlines the 16 types of personality combinations you can arrive at. Best of all, it derives a personality summary from each combination. For example, here's ESTP, the extrovert who needs sensory motivation, thinks more than feels, and perceives more than judges:
Tolerant and flexible; actions, not words; the doer, not the thinker; spontaneous; implusive; competitive.
It's much easier to dream up a character, for some writers, if you can peg that person on one of those 16 summaries. Best of all, since the basic types have been summarized, it helps get the plot-first writers motivated about characterization. The summaries and the types are an easier step up into someone's head. You have to take this step to make a strong character, or at least one who makes sense when they act.
That's an important step to get your writing into someone else's head: the reader's. "Oh yeah, I know somebody well who's like that" is the kind of connection you want readers to make with your story's actors. A plot can be brilliant and lure a reader to the story. They stay more often, and bring away more from their story time, when the actors are memorable.