There's plenty of questions here on multiplayer and questing, but I can't seem to find any answering this particular question.
I just joined my brothers game to give him some items, and afterwards I have to kill Belial again.
When resuming my own game, I was back at an earlier point in my quests, so I left the game, and selected the item to pick my quest start, and I could not pick a point after killing Belial.
Is there some definite information how quest progress is influenced due to multiplayer?
For instance, how can I know, say after killing Belial, that I'm at a particular point where it would be safe for me to join a player that hasn't progressed as far, without me loosing any progress myself?
Best Answer
It's a little counter-intuitive, but the best way to tell is actually waypoints. When you abandon all question progress, you lose your checkpoint, location, etc. - any progress you may have made on your current quest. The consequence of that is that you'll need to go back to the last waypoint the next time you start that quest again.
There is a stipulation to that - for Act bosses, there is someone you talk to after defeating the boss to actually complete the entire quest. For example, after killing the Act I boss, there's actually a second quest before the next act (which consists of "talk to the person who takes you to the next act, and go to the next act").
I had this happen to be when I defeated the Act III boss - I recommend always either finding a waypoint or going to the next act before losing any current quest progress.