By digging deep I mean going beyond the basics. Past physical descriptions, GMCs and their environment.
For me, the very core of character comes from the M in GMC, the character's motivations. I delve into that motivation to figure out where that "Why" came from.
Was his mother an alcoholic? Is that why he has an aversion to meeting women in bars or to dating women who partake in even social drinking? Even subtler clues can provide great fodder for personalities, phobias and tastes. Maybe his childhood friends made fun of his hand-me-down clothes. Would he grow up to be a fashion plate?
I find the more I know about a character's childhood, early dating life, school experiences, parents and siblings, etc., the easier it is to create a real, non-cardboard person.
What about you? Do you have a certain way of mining your characters to bring them to life?