When you're writing your book or novel, how much back story should you put in for each of the characters? The general answer is as little as possible, but in reality it all depends. If you're introducing characters in to your story, you don't have to tell us what time they were born, who cut the umbilical cord and which butt cheek got slapped by the doc. If it has nothing to do with the story, and nothing to do with building or developing the character, then leave it out.
I'll give you a great example. Years ago I wrote a script called "Then Genesis Project." It was the third script that I'd ever written and I ended up selling it. Before I wrote it, I studied serial killers for three months. It was both interesting and disgusting to reading about all of these human monsters and their horrifying acts over the past hundred or so years. I even read the history of torture. Not great book club type material, but it gave me the information I needed to help understand the character I was about to write. A lot of the research that I read discussed possible motives for the heinous crimes these men and women committed. Most of it had to do with either nature or nurture situations. I thought it was all interesting, and so of course I felt the audience would also be interested in knowing how and why certain serial killers came into existence.
I included back story into my story. I gave grizzly details of a handful of notorious serial killers, like Ted Bundy, John Wayne Gacy, Jeffrey Dahmer and Charles Manson, but I also included a little history on some not so known murderers too. Needless to say, when the first draft of the script was finished, it was nearly 200 pages. Something needed to go, and the more people read it, the more I realized that I had to slice the killer background out of the story. Since most of the serial killers shared a common background; molestation, physical or emotional abuse, killing or torturing animals, etc...I could easy combine all of the killers back stories into one succinct glob of grossness. I was eventually able to cut it down to a few key points and leave it at that.
Now, as with most rules, there are exceptions. If however you're writing a novel series, or a TV series, or even film series, and you have a main character that your audience has been following for years, then back story may be what they have been salivating for the entire time. Imagine if we had no idea where Superman was from, no clue as to who he really was, how or why he came to be? The audience loves the character and has been following Superman for years like a pack of hungry dogs, just waiting for the writer to drop any scrap of back story that could answer some of the hundreds of questions they might have. In this case, back story can be used to tease the audience along, keeping them intrigued, which translates into built in audience and hopefully book or ticket sales.
So, when it comes to using back story, a little can go a long way. Just keep in mind that no matter how much back story you choose to put in, it needs to either help build your character, build your story, or tantalizingly tease your audience along. People read or watch entertainment to be entertained, not to be educated. There are books and films specifically for educational purpose, and your audience will decide which they're in the mood for and seek it out. Entertain, or educate, trying to do both deludes each and tends to bore your audience.