In a video game, we can't do that - but more importantly we cannot, in most cases, even suggest through dialogue that we don't altogether buy what they're telling us.
Actually, video games make this sort of contest all the more possible to do well. You can have double-blind skill checks, Gale could roll a Deception check, and Tav an Insight check without the player ever knowing; with the result being folded into the dialogue without fanfare. One of the few instances roleplaying is better with a computer.
That isn't to suggest Larian is doing anything like that, but something to keep in mind.
Calling someone out on there self-reported backstory, I'm not entirely with you, especially before it has serious bearing on you or the adventure...I'm of two minds.
Gale could go through a heel turn without his first statement being untrue, it could be something he believed at the time, that would be true for a PC or NPC. I'd accept that of Tav, though I'd appreciate a little narrative coherence between those two states. Without it, it becomes ad hoc storytelling, very dissatisfying.
Shadowheart is brainwashed and is an otherwise heroic Selune worshipper, who is also probably Selune's chosen
I loved you Shadowheart! You were supposed to bring
balance to the Weave! Not leave it in darkness!
I'm sorry that's all I could think of when I read that.
