In The Order of the Phoenix, just when Dumbledore’s Army was discovered by Umbridge, Marietta responds in a manner which allows her to save the DA from most trouble... but her eyes are strangely blank.
Everyone in the room was gazing at the top of Marietta’s face. Only her eyes were visible between the pulled-up robes and her curly fringe. Perhaps it was a trick of the firelight, but her eyes looked oddly blank. And then - to Harry’s utter amazement - Marietta shook her head.
Even later, we see this:
Marietta was standing exactly where Umbridge had released her. She seemed neither perturbed by Umbridge’s sudden attack, nor relieved by her release; she was still clutching her robe up to her oddly blank eyes and staring straight ahead of her.
So, was Marietta Confunded or Imperiused? We know from The Deathly Hallows that one can use the Confundus Charm to bend someone to their will (i.e. Snape giving the seven Potters idea to Mundungus). Was Dumbledore or Kingsley doing this? If not, why were her eyes so blank, and why was she so unresponsive towards Umbridge’s attack?