I've been exploring a novel I started long ago, changing it from first person to third. It's going surprisingly well: The point of view is still that of my hero, an aging US Marine general in 1935 who is very much a child of the late 19th century. So I'm getting away with a slightly old-fashioned style that suits him.
This is an alternative-history story: a disaster has effectively ended the British Empire early in the 20th century, and the world is of course quite different by 1935. I remembered having written a summary of the history of that world, and dug it out of an old file. It may need a lot of changes.
But it brings up a point worth thinking about. Your novel, ideally, begins as close as possible to the climax, at the moment when the protagonist is launched on a fateful trajectory.
Think about the classic western High Noon. It starts just a couple of hours before the climax, when the marshal is getting married and about to start a new life. We don't need to see his courtship of his new bride, or his arrest of the gunmen who are now out of prison and coming to take revenge. That gives a huge intensity to the events between the morning wedding and the high-noon arrival of the gunmen.
But we always need to know the backstory. You should have at least a summary of what's been going on in your characters' lives before the incident that triggers the story. But it can take other forms as well.
One way to do it is to write your protagonist's resume, though not one that he or she would ever submit to an employer. It would include all the routine resume information, but also include items such as hangups, fears, medical history, sexual preferences, and philosophy of life in a single sentence. You will be amazed to find out what's really going on with your characters when you fill in the blanks of such a resume.
Another way to deepen the backstory is to write an account by your protagonist (or antagonist) of the worst thing that ever happened to him or her. I did this for Mike Henderson, the rather taciturn hero of Henderson's Tenants. It turned out to be his wife's desertion when he came under pressure from the government because nanotechnology had suddenly become the Politically Convenient Threat of the Year.
And the way Mike described that desertion—to my surprise—told me he was both furious about her desertion and bottling up that fury. That gave me ideas for events that would reflect his feelings and his efforts to deal with them.
So spend some time working out not just your plot, but the origins of that plot: What your characters want to achieve, why they're choosing particular ways to achieve it, and how it will lead to the trigger event that sets off the novel itself.
Don't expect to put the whole backstory into the novel itself. Key events will be useful exposition as flashbacks or as topics of your characters' conversations. Much of it, though, will be just stuff that you know about your characters but don't need to put into the story itself. Tolkien knew a lot more about the history and culture of Middle Earth than he ever told us, and I'm glad he didn't. We know just enough.