I also love it when my characters surprise other characters. I think the best example is the Fairy Queen allowing Eileen to defect which stuns Eileen herself, who had assumed she would just be kicked out. It doesn't please Brankaresh, the Chief Wizard, either who also had assumed Eileen would be kicked out and though he would always prefer Eileen to die, horribly ideally, had accepted exile as the next best thing. Surprise actions like that (but logically justified and the Queen's actions are sensible) propel other characters into doing/saying things they hadn't planned to do.
If you generally know how your characters will react, good, but they should be capable of going off on a limb (with justification) as we all do in life from time to time.