What would happen if you don't talk to someone until post ga

I probably won't do it. But I do recruit everyone and just not take some of them out much if at all. Only time I take out Sera is for her side quest. Then I pass her off. First time I did it she had her temper tantrum.

First run I went the majority of time at Skyhold not speaking to Cassandra because a bug made her invisible most of the times I went up that part of the castle. If a patch hadn't remedied the situation I would have missed a lot of important dialogue with her, not to mention the whole Seeker quest. I almost missed talking to Leliana at Haven for pretty much the same reason. She didn't seem to be in that tent at first and so I only became aware it was the place to find her by accident. Not talking to Leliana or Cassandra is potentially far more serious than not talking to Sera because of the whole implications for the Divine decision. I suppose you could say the same about Vivienne but it would seem to me you are far more likely to get her as Divine if you do talk to her. I ignored her much of my first run after Haven because my mage really didn't like her and so I wasn't even aware she was a potential candidate.

I must admit though that I don't see much point in not talking to the companion at all, unless it is accidental. If they really don't interest you, then simply don't recruit them. As most of the companion quests need to be completed before the Arbor Wilds and definitely before you take on Corypheus, I would imagine that not talking to them would result in either minimal dialogue in Trespasser or the same as if you had had a hostile relationship with them.