After River's projection says she was the one to open the door, she walks to the right hand side. At this point no one except Clara can see her, however although he is out of focus, you can see Dr Simeon's eye-line follow her as she disappears off camera. See more
This man must fall, as all men must. The fate of all is always dust... The man who lies, will lie no more. When this man lies at Trenzalore... The girl who died, he tried to save. She'll die again inside his grave.