I think light puppeting by GMs is okay assuming how well GMs know their players and trust them and vice versa. If it's a natural reaction to something that's not out of character, I don't see an issue. It's when you start making serious actions for other characters or fucking around with combat where things get sketchy. 

For example, if you and another player's characters are walking and talking, I don't see an issue with saying the other character took something if offered by an ally or walked through a door that was held open if it's minor stuff like that. If it's something like a fight or another scenario that requires player input, you should never assume anything and let them do it. This is why collabs are great.

But mostly, never control another PC or assume you know their intentions.