To make your characters come to life, you need to understand their lives before your story begins. Part of fun in writing is getting to know your characters. One of the ways is to prepare character sketches of the main characters. This should be more than just a a visual description of the height, weight, build and hair and eye colour, but should cover psychological elements such as their desires and pattern of thinking, as well as their weaknesses and quirks. Some writers suggest going even further: building their family history, their schooling and career development. I’m not sure I would necessarily agree with going that far; I tend to keep my character sketches relatively brief. However, life events that have moulded the character into who they are today are clearly important. Another way to discover your characters that has been suggested is to conduct an interview with your character to see what they have to say about themselves and what they believe in. (All good writers are good at talking to themselves!) Alternatively, some writers may prefer a more organic way of getting to know their characters through the process of preparing the first draft. They may well not be fully formed at the commencement of the story, but they will by the end. Whatever, method you use your character has a backstory and it will affect how they behave.
However, while a character’s backstory is important to the writer, it is of less concern to the reader. That’s why it’s called backstory: it happened before the story begins. A new writer, having done all the hard work of researching a character’s backstory, should resist the temptation to simply dump the backstory on the reader at the first opportunity. Donald Maass in “Writing the breakout novel workbook” suggests finding any scene in the first fifty pages which brings players to the stage, sets up the situation, or is otherwise backstory and cut and paste the material into chapter fifteen. Then to review the material in chapter fifteen to see if it cannot be cut outright, or if not to find the best place after the mid point of the novel. Strong advice. But those first fifty pages are so important they shouldn’t be burdened with an information dump. Information about the character’s history that is vital to the plot can be drip fed into the story. Sometimes holding back information can even be helpful in raising interest in the reader’s mind why is behaving that way?
So, backstory is important; but not so important that it has to be revealed to the reader, at the earliest opportunity.