I try and give them a reason for doing it - character based, history based, and then try to lead the audience through that journey to that point. Also, as the_rck pointed out above, nobody is all one thing all the time, and canon tends to leave gaps for extrapolation and possibility which can be effectively used.

And then, to some degree, exactly how un/characteristic a behaviour or action is depends on your audience and how willing they are to be led where you're taking them.