Though most of the really good personality tests are expensive, there's little point in shouldering the (often considerable) expense when you're interested because you're a writer. It would be time-consuming and probably frustrating to take the same test over and over as different characters. Plus, some of the tests would pick up on the fact that you're not being truthful about your own personality, and that would confound the results.
I've found I can do a lot with a list of personality qualities that will tap important aspects of my characters, though, and I do like to use characteristics identified as important by personality tests. Typically a great deal of research (usually involving factor analysis, which allows researchers to group similar personality traits together) has gone into choosing personality factors that will be revealing.
As I said, I don't use the tests themselves—instead, I use descriptions of high (or even better, high vs. low) scorers with a list of identified traits. I really like the Personality Research Form (PRF), though I have also used the Myers-Briggs, the Keirsey Temperament Sorter, the 16PF, the California Psychological Inventory (CPI), and the NEO-PI.
Of the tests listed above, the Keirsey and the Myers-Briggs are my least favorite (which of course means they're the ones that are both easiest to find information on and free) because they only provide information on four personality dimensions: Introversion vs. Extraversion, Intuition vs. Sensing, Feeling vs. Thinking, and Perceiving vs. Judging. Though both tests measure the same things, they provide results in different ways. The Myers-Briggs results in a 4-letter code like INFP (Introverting-Intuiting-Feeling-Perceiver) or ESTJ (Extraverted-Sensing-Thinking-Judger); the Keirsey puts the results into one of four quadrants (so the testee is said to be a Guardian, Idealist, Artisan, or Rational).
Of the remaining personality tests I listed, the NEO-PI is probably the one most in keeping with modern trait researchers' understandings of personality, since it looks at the "Big Five" Dimensions of personality: Neuroticism, Extraversion, Openness to Experience, Agreeableness, and Conscientiousness (each of which can be broken down into 6 subcategories). I do like the Personality Research Form, though, because it allows you to look at character qualities like Harm Avoidance, need for Social Recognition, responses to Change, and how much your character likes to Play or do things "just for fun." (You can see a description of high scorers for each of the 20 PRF factors in this sample report.) Also take a look at the 16PF, which is very similar and looks at characteristics like Rule-consciousness, Self-Reliance, and Perfectionism. The Wikipedia entry on the test also offers descriptions of both high and low scorers (which may be helpful in constructing your own scales, as noted below).
As a writer who's just trying to better understand your characters (and their similiarties and differences) here's really no reason you have to stick with one personality inventory. You can cobble together the qualities you're most interested in using to look at your characters. Whatever you decide, the trick is to consistently describe your characters using the same "inventory" or "tool."
After you've become familiar with each of the personality factors in the inventory you want to use (for the examples below, I'm using the PRF), one approach is to construct a Likert-type scale for each personality quality, something like this:
Hates Change (Steadfast) 1 2 3 4 5 6 7 8 9 10 Loves Change (Capricious)
Low Harm Avoidance (Hotheaded) 1 2 3 4 5 6 7 8 9 10 High Harm Avoidance (Apprehensive)
In the two examples above, I chose a single synonym, which I placed in parentheses, to help define each approach. You might choose a different synonym, or eschew synonyms altogether.
Another approach is simply to list the personality factors that are high in each character, placing heavy emphasis on one or two. If your characters are well-defined enough, you'll find that one character might value Endurance ("persevering...relentless...dogged...durable") while another lives a life of Impulsivity ("volatile...hasty, rash, uninhibited, reckless"). Just be careful not to emphasize a particular quality so much that your character becomes a one-dimensional cardboard cutout!
Once you've put together your character profiles, it's time to start thinking about how your characters will show (are showing, or have shown) their core personality traits in action. Since personality can be defined as "characteristic patterns of thought, emotion, and behavior," it's important to consistently show the character's personality shining through each thought, emotion, and behavior. In other words, you must work the details that define your characters into the story. You must show character rather than tell us about it. That is, infodumps and declarations of character ("I am an impulsive person") are be redundant at best. At worst, they can backfire.
Think about it—if someone you worked with ran around telling everyone what an honorable guy he is, but you never saw him do a single honorable thing, you'd think he was a liar, wouldn't you? Readers will feel the same way about characters unless they get to see that honor in action.
© 2012 Carolyn Kaufman, PsyD ♦ Psychology for Writers on Psychology Today
Carolyn Kaufman, PsyD is the author of The Writer's Guide to Psychology: How to Write Accurately About Psychological Disorders, Clinical Treatment and Human Behavior. More information is available on the book's website.