For well-rounded characters, I always try to think of something that the character wants or thinks or does that may not be in line with what the reader will imagine the character to be. We all have preconceived notions we bring to characters we read, so I try to think around those. One very simple example is Billy Dent, the serial killer father from my I HUNT KILLERS series. We have a notion of what serial killers are like, so I swerved away from that, made Billy avuncular and fun-loving and verbose.

I think when you show a reader a "type" of character and then peel back a layer or two to show something unexpected that those are the sorts of character readers remember!

I talk a bit more about this in some writing advice blogs I wrote a few years ago. Check 'em out here, if you like!