I am totally new to RPGs and haven't participated in a single game yet. However, I'm interested in the topic and today, I found a link to Dungeon World on this site and started reading.

There's something in the introduction that I need some clarification about:

As you play, the players say what their characters say, think, and do. The GM describes everything else in the world.

Does this mean that a player who plays a character with a hidden agenda will say things like <weird voice>Oh, you're going to sleep? Hee-hee, good, easier to poison you when you're asleep!</weird voice> in addition to saying <other weird voice>Oh, ok, good night then.</other weird voice>?

Is that something that happens in most RPGs or is it more specific to this one? Other questions on this site mention interesting stories involving a player whose real character is only known to that player and a GM, but I don't know whether these are rare things that some people come up with or whether it's a common thing.