One of the most important things, too, in developing characters is making sure all of them have a backstory but not necessarily revealing each character’s backstory until the last possible moment; and sometimes just a few words are needed. I think some of the most effective backstory techniques are those that hint rather than tell out right–leaving just what we need to know. Since backstory is never the point of the story, rather something preceding it, sometimes there’s a risk of deviating from the plot. So I’ve been working, for example, within a location (New Zealand) for my main character, and what the character is experiencing, which is more subtle within the first-to-early mid-sections of the novel I’m working on now. But toward the end of the novel, there’s more of an opportunity to see the main character purposely revisiting the location in terms of the novel becoming more full circle.
I started out needing to develop the main character, secondary characters, and supportive characters’ backstory by writing general statements about each of them, which helped me to keep on target with the direction of the novel. And, of course, always an outline helps–which can configure into helping sustain backstory.