Having heard from AZ that his mind is going to be wiped, Fives escapes the custody of his fellow clone troopers, taking AZ with him and stealing the chip that was removed from Tup's head. Later, they sneak back into the medical facility in order to "continue our research," as AZ puts it. Convinced that there's a chip inside his own head, Fives tells AZ, "You have to take it out." However, AZ is hesitant and tells Fives that it's a "dangerous surgery" and "you may die," to which Fives replies, "It's a chance I have to take. I trust you."
Throughout the episode, AZ's mouth lights up when he's speaking. But after Fives tells him, "I trust you," it lights up without his saying anything and stays lit for a few seconds before he finally says, "All right, I will do it."
Because it's been established that a lit mouth is a speaking mouth (in other words, an open mouth), AZ's mouth being lit here without his saying anything is the droid equivalent of his mouth hanging open in surprise.
