By writing whatever comes to mind for at least twenty minutes. I find that's where I get the random thoughts that appear in my writing, such as in UNFORTUNATES, Vanessa wonders if she knew she'd get a second chance at life ahead of time, would she be more reckless. Or even the completely off the wall, random remarks like why are the specks of black in your vision called floaters, or if boys know that we can see everything through sweatpants 99.99% of the time.
I think these thoughts make the characters real because, while we may not say it out loud, we all commentate our lives as we go about our business.
While it may not be stellar writing on the initial run, you can use the ideas you accumulate during this exercise to work towards a fleshing out characters.