In my experience there are more or less three stages:

1) Flirting without physical contact, or only with physical contact that might be acceptable for an acquaintance (brief touches, possibly friendly hugs) leading up to some kind of asking-out or other fairly direct "are you interested in me" question

2) More overt flirting, possibly later-stage physical contact, possibly leading up to kissing

3) After kissing or similar-level contact, if things seem to be getting hot and/or heavy, body-language-only communication halts. Serious Discussion is had about What Will Happen Next, including sex and/or Future Plans. This is fairly explicit and consists of things like "Do you want this to be a serious relationship?" and "Do you want to do sex act X?" and "I need to tell you about Y."

I doubt this is really applicable to anyone else, because our culture doesn't seem to really have a script that is standardized enough for anyone to follow, but it's a script that I like pretty well. I've skipped steps, but more or less always follow the "Talk about it explicitly once physical contact reaches a certain point" part, and I think it is helpful.