While obviously you should know your character as well as you can, they should still be able to surprise you with what they say (especially under times of stress when behaviour can be known to change in people). They should also be capable of surprising you with what they can do. I've been surprised by people in my time turning out to be more sympathetic to "my cause" (so to speak) than I'd realised.
So let your characters surprise you, it is their story after all.