Character backstory is an accumulation of the events and character development that chronologically precede the narrative of the novel. For the purposes of this article we are focusing on the backstory of the protagonist, however, character backstory is just as important for your whole cast.
A character's backstory is an essential part of explaining motivation and reasoning behind actions and reactions, to the reader. It is also a way for the reader to build a connection with the character. Intimate knowledge of character backstory helps the writer build a convincing character arc and their development towards the final goal. This in turn helps to keep the plot focused and moving forward.
A great deal of work goes into backstory – almost as much as the story itself, but it should be work that mostly remains behind the scene and isn't written out in full in the novel. Backstory should be seamlessly woven throughout the novel whether through dialogue, internal narrative, implication through actions and reactions or flashbacks (used sparingly).
The first piece of advice we give to authors before they move forward with their writing is to write out character backstory in full for each character. Don’t however look upon this as rigid - as your plot develops, so too do the characters and in ways you possibly hadn't planned. Always allow yourself the luxury of going back to backstory and making small changes and building on what you have as the characters develop. If you find yourself making big changes, it may be that you don't yet have the character cemented in your own mind - in which case, this will be a very beneficial exercise for you to follow.