I laughed when I read the OP saying "it sounds like we're not close if all we talk about is business", because my mom and sister are like, joint at the hip and yet all they ever talk about is what they're cooking for dinner and the new lamps/cushions/other pointless interior 'must-haves' they've bought recently. Those are things I'd consider 'neutral topics' that are safe to talk about with people you don't know very well, but they're dull, bland, boring and nothing I really care about so I'd be hoping to move onto the 'real conversations' when I get to know the person better. Yet they are perfectly content to talk about that stuff all the time, and they are close.
My INTJ friend and I talk about all sorts of things. Really there is no limit to what we talk about, but it's rarely people - well, not individual people (not gossip! lol) but we do talk about people as in 'society' quite a bit. And science and history and places to go and vacations and all sorts. Quite often we're just talking about mundane, day to day stuff cos that's what we're doing together so the talk is sorta 'on topic'. But we are very close - occasionally we talk about our problems together, in a very textbook NT kind of way, y'know, describing and defining the problem, analyzing it and working out solutions that make sense.
In the years we've known each other we've definitely influenced each other - he's made me more organized and I've made him a bit more laid-back and talkative