You can become a Hand or an Eye in the endgame regardless of what you say or do throughout the rest of the game. In order to become a Heart, you need to get a flag set over 100. (The flag is (100,6), for anyone who wants to edit scripts.) In general, only what you say directly to Redbeard appears to affect the flag. You can say anything you want to anyone else, and it generally makes no difference. Also, in general, telling Redbeard what he wants to hear increases the flag, and defying him decreases it.
There are tons of these, so I can't list all of them. However, basically everything that you can say or do that acknowledges Redbeard's authority, calls him strong, provides him the backing that he wants in any given situation (staying silent when Redbeard argues with Deniz at the Green Refuge, backing him up verbally or by being ready to fight when he confronts Callan), or suggests that you will be harsh with Farlanders and rebels adds between 3 and 5 to this flag. Any equivocal answer ("I'm just following orders") does nothing. (Anything said to anyone other than Redbeard, as long as Redbeard isn't present, also does nothing, as far as I can tell.) Anything that isn't what Redbeard wants to hear — telling him he's weak, he should show mercy, etc. — subtracts from the flag.
There are a few moments that you might expect to impact the flag that don't. For example, your advice on Callan's final fate doesn't impact the flag in any way.
As far as I can tell, you can kill just about everyone with impunity.
* In Fort Foresight, you have the opportunity to attack Envoy Dirran. The text around doing so gets pretty dark, but all that happens is that Dirran is replaced by a different person — Envoy Vega — who says the same things in the endgame.
* I can't find any consequences at all of killing Hand Taroe in Fort Foresight.
* I can't find any consequences at all of killing Zhethron.
* Obviously, killing Velusa has consequences in the Temple of Velusa, but as far as I can tell, there is no additional benefit to killing Velusa that you don't get from challenging and then asking for forgiveness from Velusa (other than dropped items). Hand Hahn rewards you the same way.
Obviously, giving Redbeard poison and giving Dirran hair from Redbeard make the final battle easier if you choose to fight Redbeard. I don't see that they significantly impact the ending otherwise, though.
It's not clear to me that anything else matters, in terms of restricting your endgame options.
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