* In Actor stance, a person determines a character's decisions and actions using only knowledge and perceptions that the character would have.
* In Author stance, a person determines a character's decisions and actions based on the real person's priorities, then retroactively «motivates» the character to perform them. (Without that second, retroactive step, this is fairly called Pawn stance.)
* In Director stance, a person determines aspects of the environment relative to the character in some fashion, entirely separately from the character's knowledge or ability to influence events. Therefore the player has not only determined the character's actions, but the context, timing, and spatial circumstances of those actions, or even features of the world separate from the characters.

In most of the stance-discussions, we've considered players rather than GMs because the player:character relationship is usually 1:1 and very intimate. I think that GMs employ stance too, however, that discussion awaits development.