Another Vader "assembly" mistake. At the beginning of the scene, you hear Anakin screaming in pain from all of the robotic parts that are being attached to him & he moves his "good" arm around & knocks over a droid & I believe a tray. If you look at his face, you will notice that while we can hear the screaming, his mouth isn't moving. He isn't even grimacing in pain. While it may be argued that he couldn't move because of his burns, in scenes immediately after this one, you see that he can actually move his face. He is also able to speak, evidenced by him answering the Emperor with his by now famous "Yes, Master".