I've been trying to get every conversation achievement in this Diablo III run, and I MISSED ONE right at the start of Act III. Major bummer. Might have to do another character to get that.
I don't remember if you have to beat the game first, but you can go back to previous chapters just by picking it. In fact, some of the conversation achievements can only be gotten by talking to characters at points in the game you shouldn't normally have access to them.[/quote]
Yeah, you were right. You do have to beat the game first, but you can go back in the mode select menu. I found that out and did some Achievements cleanup today, then ran Grifts for a few hours.