I've read many articles and attended workshops where the issue of misuse of character backstory has been discussed. General consensus is: too much, too soon can be the kiss of death in a novel. I've judged enough contest entries to agree. When I read three paragraphs in the first page about little Sara's childhood and how she witnessed her brother dying and how the trauma of that affected her so much that she couldn't cope with opening her heart up to another person and on and on, the pace of the story is immediately killed and the present day conflict is put on simmer on the back burner for so long I totally lose sympathy and interest in reading more about the character. So, no question that too much backstory too soon is not necessarily the best way to let the reader get to know your characters or to get invested in your story.
However, that is not to say that backstory isn't one of the single most important things to develop as you plan for and, eventually, write your story. Especially for secondary characters.
Like in movies, supporting actors can often elevate a good movie into a great one. In books, secondary characters can add more depth and understanding to the main characters and to the themes and plot line of the book - if they are well-rounded. If they have a backstory.
I've been tackling the revisions to Illegally Blonde this week and one of the main things I needed to work on was bringing a bit more depth and understanding to one of the secondary characters. My heroine, Lucy, has a cousin who's not exactly a nice person throughout the book and there are definite tensions between the two of them. My fabulous editor suggested that there needs to be a better resolution to their relationship by the end of the book and asked some interesting questions about why this cousin did or said some things and suggested some potential rationale for her actions. And my reaction was not, 'Oh, migod. WHY did she do that? How can these two reach an understanding when they've spent most of the book at cross-purposes? Who is this person?' Instead, I rejoiced when said fabulous editor's suggested rationale was exactly what that character was all about. How did I know this? Because I already knew her backstory.
Even though this cousin was a secondary character and I perhaps hadn't fleshed her out as much as I should have I did know her backstory well enough to be able to answer the questions my editor had about her. And even though she is unsympathetic I never once disliked the character because I KNEW why she was acting the way she was acting - even if the reader didn't. I knew my character and I knew her backstory - even if I hadn't used it. Now, what I have to consider in doing this revision is where to drop a few little nuggets of the character's backstory in earlier so that the resolution is not considered totally off the wall. That way when a certain event happens to my heroine near the end of the novel which triggers the revelation of the cousin's rationale for her actions throughout the story her response is believeable and understandable - even if it has been the opposite of the way she's been acting all through the book.
I know I'll work it out one way or another. But what I won't do, for sure, is dump the cousin's history in all at once the first time she is introduced to the reader. Backstory is best handled like a particularly potent spice. Too much all at once kills the whole flavour of the dish. A little dash here and there, at the right times in the cooking process brings out the best flavour of the food. Or here's another, non-food analogy. Just think, if you go out on a first date, do you really, really want to know everything that's happened to your date in the last 30 years? A little bit of mystery, a bit of information, a few interesting anecdotes, leaves you wanting to hear and learn more about that person. So it makes you look forward to the next date. Relating this to a book, you read on to Chapter two.
I guess what I'm trying to say is that I believe backstory for your secondary characters is equally important as knowing your plot inside out or what main themes you are trying to convey in your book. Backstory is what makes every character unique and interesting and understandable. People you either root for - or not. Depending on their purpose. Every character should have a purpose and some element of history to them. Remember that old quote: "There are no small parts, just small actors?"
But a word of caution. Be judicious in how you share that backstory. You don't need a complete geneology for the newspaper boy if he's just delivering the paper in Chapter Five. But if that newspaper boy is going to rip out sections of the paper or deliver it late every day because he wants to torment the old man who looks forward to reading it as the only highlight of his otherwise boring day, it might be good to know WHY the newspaper boy is doing this - eventually. Determine how important a secondary character is to the plot or sub-plot of your book and the impact that secondary character has on the main characters of your book and plan backstory accordingly and determine whether and how it needs to be revealed.
Like everything in writing, this is a balancing act. One I still work on every day. And what I'll be working on now in this revision.