I tend to burn out developing my stories before they even reach the page, so I'm taking some advice to pants a novel. I didn't quite follow the advice to the letter; I used an idea that had been rolling around in my head for a while. Character A had a backstory, Character B was in conflict with him, and there would be an antagonist to both of them. Anytime I try to introduce the antagonist, however, I find myself losing interest because I have no idea who he is. I'd be happy to throw him away and see where the story goes without him, but I'm concerned that it would lose much of its motivation.
I need to know all of my main characters better. I recently wrote a scene that shed a lot of light on Character B's reasons for wanting to leave the situation in which Character A has placed him. That and Character A's backstory might be enough to provide the motivation I'd miss if I chucked the antagonist. I would like to write some scenes revolving around these details, but they wouldn't be part of the finished novel and I'm afraid of slipping into procrastination.
Is there room for writing scenes that could prepare me for writing the main story, but won't be in the final version of the novel? Is this procrastination, or is it useful for a story I haven't spent a lot of time developing? How can I tell the difference?