You'd be surprised by how much insight can be gained just by observing a character's thought process from the outside followed by their actions, I think, meikle. You just have to trust your audience will put in the effort on their side to glean that information without having it spoon fed to them via exposition.
Unfortunately, trusting their audience isn't something TV/movie producers do anymore, at least not the majority.
A Feast For Crows quote below, maybe spoilers:
"No bells will ring for you, Tyrion," Cersei thought. "I will dip your head in tar and give your twisted body to the dogs."
I don't think you're going to get that from watching a person's face. Their state of mind, their emotional state, sure... the internal monologue that a lot of scenes in the book draw on? Not so much. Tyrion isn't even present in the scene that that quote is taken from. GRRM is often very explicit about showing exactly what his characters are thinking, not just suggesting it; he uses, essentially, voice-overs in the books themselves. It's easier to get away with in text, I guess.
edit: Though Cersei does say a lot of terrible things out loud that more or less match up with that quote. "If you ever call me sister again, I will have you strangled in your sleep." So maybe we can just assume she's always thinking awful things about everyone.
God, I love Cersei.