In case you don't know interjections are dialog between your companions, or a line your companion might say in a conversation you're having with another NPC. This is one of the things I find most disappointing about Pillars of Eternity when comparing it to Baldur's Gate 2.
Relegating interjections between your companions to short, and mostly irrelevant, conversations while you're moving on the map is not the best way to do it. It's not cheaper, because every line has to be voiced unnecessarily, and it forces the writers to keep the dialog as short as possible. A character like Jan Jansen would be impossible to recreate with a system like this because of those limitations, and it was personally one of my favorites in BG2. It also presents the problem that the only way to do character development is to make the player talk to their companions where there are no restrictions for the text's length, because whatever they say to each other with the current system is usually not that interesting since the writers have to keep it as short as possible.
The lines they have in regular conversations are also quite disappointing. The vast majority of them feel out of place, and it doesn't feel like they are actually taking part in the conversation. Several times they will insult the speaker or make some nasty comment about them, but there are almost no reactions to those comments. It feels as if everything they say is just a whisper only the player can hear. Like the writers never made the effort to make it seem like your companions are participating in the conversation and being an active part of the group. There are ways to make this work without having to write different dialog for every NPC depending on which companions are in your group, BG2 did it already.
This conversation is a perfect example (ignore the first Jaheira line, that one is supposed to be the player):
To the player, it looks like Jaheira is taking control of the conversation, and asking a question to the dryads. Compare this to most interjections in PoE:
Edér here might as well be just a voice in your head, since that conversation clearly doesn't feel natural at all, that's not the way those NPCs would have continued talking if Edér had actually said that.
Let's go back to the BG2 example. The great thing about it is the dryads aren't actually reacting to you having Jaheira in your party. Jaheira is the one reacting. It is an illusion. Cania will say that line regardless of if you have Jaheira or not in your group and the line feels natural in both cases. The key here is that, if you happen to have her in your party, you'll get a better experience, thanks to that illusion. Combine those illusions with dialog where they actually react to who you have in your party (remember Minsc talking to the kid from Umar Hills, for instance) and you'll have a game that will seem way more reactive that it actually is, which is one of the reasons we love BG2.
I'm also not the biggest fan of endless conversations with your companions asking 20 questions to them 5 seconds after they join you (Durance is especially bad at this), because it slows down the pace of the game considerably. But Obsidian apparently loves that since they do it in most of their games, so that's not likely to change. But there are better ways for your companions to talk about themselves without making it seem like the player is interrogating them.
Thoughts?