Personally, I try to keep stories as "tight" as possible. I do develop my characters to the best of my ability, but when it comes to their pasts and the reasons why they're the way they are, that tends to fall by the wayside because it's not really relevant to what's going on in the plot. But I do think about it, and I often delve into the backstories of the characters even if I don't actually explore them in the actual comic.
My characters in Biophilia are full of stuff like that:
-Pedro had a very neglectful and unsupportive mother who would spend much of her time berating him. As a child, during a visit to the zoo, she wasn't at the place she said she'd be when he returned from the toilet. He wandered off a bit, distracted by the animals, and when she found him she yelled at him in front of a bunch of people, disregarding the fact that she was the one who wasn't waiting at the predetermined place. Pedro felt thoroughly humiliated by this and never really forgot that day. His only good family relation is with his grandmother.
-Shigeo was raised by his mother to be successful and nothing but, even at the cost of everything else in life. She taught him to completely ignore other people and dehumanize them in order to get rid of emotions that could hinder him in fullfilling his full potential. Shigeo's father was vehemently against this, and this deep rift in opinions led to their divorce.
-Natalia knew a person who commited suicide in elementary school, and it had a profound effect on her outlook as she grew older. She would never turn her back on someone she knew was in distress after that, and she always tried to give people the benefit of the doubt, even if they were colossal turds.
-Mihail is an only child, and his father wanted to raise him to be tough and masculine, which didn't go well with his more introverted, meek character. Mihail trained a lot of martial arts as a child and is quite good at defending himself. He is also very lean and muscular, but doesn't like to show it because he feels his body looks brutish and doesn't reflect who he is inside.
If you want to see all of those peeps in action, here's a link:
For Bright Light, the focus was much more on the characters and their history, so a lot of their past does get covered in the story proper. I don't think there's any backstory elements that won't come up at some point in the comic, so I'm not putting any of it here.
