Companion Approval and Progress

[eves_faith]

I am not yet sure if I get the approval system right. Is it crucial to get progress in the companions quests at all or only how they'll listen to me in me? Like, will the whole branch of a character be hidden from me if I never get over neutral? Are there certain crosspoints, where you have to act in their interest (apart from the obvious where they might leave?) Like for instance, in one guide it said Astarion won't progress in his quest if I don't kill the monster hunter. I just left him be oblivious about Astarion and let him alive.

Also, as Astorian does the most talking in my group, won't the decisions made with him as a speaker count or not count on his approval rating? Like, if I steal with him from someone in a conversation, will he approve it or only, if I manage with someone else?