Hi there, well in my experience (I can only speak for myself cuz all relationships are different), my SO was my best friend, so we talked about everything (almost. I mean there are things that you feel may be too just TMI )

In the beginning, usually subjects are more topical, and you probably will do activities together - finding things you have in common and enjoy. You just try to figure out if you even like each other: like being around each other, are on the same page, there is a nice comfort level, conversation is not forced, you have things in common, etc. If certain "criteria" is missing (things that are important to you), then most likely you'll (hopefully, nicely) move on or the friendship will sort of fade away.

If things are a match, and you become SOs, then you as go along, you share more and more. Like Typo said, the sky's the limit! (and you and your SO determine the limit).

*The most important thing for me in any relationship (whether it is "casual" or "serious") is that you build on a solid foundation based in truth and honesty and good communication. That way, as things up come (and esp. if you get to the "serious" part, you can face anything with strength, poise and dignity )

I hope this is a teeny bit helpful