The dream sequences only trigger after using the illithid powers. It starts with a voice that says, "Where are you?" when you're falling asleep. The first use of the illithid power is usually somewhere around Wyll when you have the opportunity to look into his mind. From there, it's just a question of how often you use the illithid options.
Raphael as a visitor triggers, I believe, after you talk to Nettie in the grove.
Astarion's bite triggers sometime after you see the dead animal along the path.
Some of the scenes trigger if you Long Rest early, like Shadowheart asking if resting is wise. But that scene will not trigger if you wait to Long Rest until you've already learned from Nettie that your ceremorphosis isn't advancing at a normal rate.
Basically, it all depends on what you do and when you do it, at least to my understanding.
Last edited by JandK; 19/06/22 01:45 AM.