I have a couple co-op complaints that may or may not be intended behavior.
First, when Player-A initiates a conversation with an NPC and Player-B mouses over the conversation, gets the "ear" icon, and chooses to listen in to the conversation (a nice feature, by the way), the initial dialogue box that Player-B receives is not voiced. Subsequent dialogue responses will be voiced, however.
I can read fine, but it's a bit disappointing that I'm missing out on some of the voice acting this way. And some conversations are only one dialogue box in length, meaning I never hear that NPCs voice (e.g. rats).
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Second, when the two main characters get a yellow "!" over their head indicating there's a discussion opportunity between them, for some reason there seems to be a hidden time limit on how long this opportunity lasts.
For example, when we read the journal from the jumper on the starting beach, an opportunity to have a discussion between the two players presented itself. This discussion also leads to a trait adjustment. However, without moving our characters my friend and I took a minute to discuss something (over voice chat) and in that time the "!" icon vanished before we could initiate a character-to-character dialogue. We ended up having to reload to an earlier save to fix this missed opportunity.
Why is it possible to miss conversation opportunities this way? It seems silly that one character can't go over to the other character at any time and say "Hey, I want to talk about that event that happened a few minutes back..".