In my early days of character creation, I had a simple litmus test. I would write a scene in a place I knew (Usually a Denny's, one with a full bar, more on that later), with my problem characters and I sitting in a booth. Denny's because they all look the same, so I didn't have to deal with setting overmuch.
You can figure out a lot about what makes a person tick by what they order at a Denny's, which is why the full bar is key. Why do they order a drink at 1AM? Why don't they? Why do they steer clear of the Moons Over My Hammy?
These days the Denny's test supplements my character research using horoscopes, Western, Lunar, and Blood Type Theory, as well as finding a character's MBTI type. That last isn't so simple, as I have to pre-select a type based on what I think would work, then cross-reference with the actual test. And even with that, while one of the characters in my current project fit into her type rather easily, the other didn't like the ISFJ type I ended up with for him.
And then the Denny's dialogue happened. And I've learned, when my characters "talk" to me (on paper, of course) there's usually a good reason - I'm going against their character.
"That's not right," he says.
So I ask what's up.
"I'm not a doormat. ISFJ's are doormats. I don't get taken advantage of." He's drinking bottled water, because Los Angeles water is gross (My view, not necessarily his. He doesn't care one way or the other). Sugary drinks aren't his thing, nor is hard liquor like the tequila I'm drinking across the booth from him. He's usually on-call, so he doesn't like anything messing with his senses.
"Interesting that of all the traits in the profile you focused immediately on being taken advantage of." I take a bite of my Lumberjack Slam (no toast). I DO love the crispy hash browns, and because this is fiction, I smear an extra helping of country sausage gravy over everything. Fictional calories don't count! Doesn't stop him from watching me ingest a heart attack on a plate and not comment. He's a smart man.
"Don't psychoanalyze me. I'm serious. I love my job, I'm good at it, and I'm loyal, yada yada. But the description even takes that and turns it negative--"
"You'd follow Margot to her grave if you had to."
"You're right, that's a negative." He sighs, and he runs his hand through his hair.
"You think so?" I take a small, tiny sip of my fictional tequila, because really, this is what I ordered it for. Sometimes, my characters and I take shots if it gets real emotional.
So this went on for about 7 pages, and at the end of it I got a pretty good sense of who he is, what he does on his off days, and why he'd follow Margot to the grave.
Over the years I've tried a while slew of exercises designed to help me know my characters (the sundry sites that force you through 100+ character questions still makes me twitch), but this is the method that's worked for me.
No method is the best, of course. I'd love to read about what works for you!