I am considering having this be a single pov novel, something I've never tried, so therefore it's worth doing. So far I have the pov character and a couple others. I had the thought last night that I don't think I can produce an entire novel with just those characters. I need secondaries, some of them relatively significant to the plot I'm still figuring out.
So I think, as I read my research material, I'll make mental note of interesting roles that these new characters can fill. Sometimes, just knowing I need, say, a paperhanger is enough to spark an idea of who the character is.
Also, when I write my exploratory zero draft scenes, I will try and introduce at least one extra person into each one, and see who works and who doesn't.