There are a ton of sites on the web offering scaled-down versions of famous personality tests, like Myers Briggs or the MMPI. Lots of sites offer free tests, but they aren’t reliable ones; in fact, most of the tests are just plain fake. Still, I think you should go and take a few.
Why take a unreliable personality test? Well, I want you to channel your character. Answer like the character would. If you can’t finish the test because you get stumped, then write down the question and think about it. Form that character firmly in your mind so you’d be able to answer typical personality test questions. Do this for many different characters. If your characters are based on real people (loosely, of course!) then answer as that real person would. This exercise will help you put real meat onto your strawman; that background investigative work will come through in your writing.
But I must emphasize again: USE THESE TESTS AS A TOOL FOR CHARACTERS ONLY. If you want reliable information about your own personality, you must see a professional who is licensed to administer the tried-and-true tests. Only those tests (that take hours to fill out, by the way) and the skilled interpretations of a professional will give you information you can count on. There. That’s my very serious disclaimer.
Now go pretend to be your characters and find their most intimate motivations!