People have a lot of trouble with what Amy was able to do in the Big Bang. What it comes down to is understanding the meaning of the Doctor's words:
"People fall out of the world sometimes but they always leave traces. Little things we can’t quite account for, faces in photographs, luggage, half-eaten meals. Rings. Nothing is ever forgotten, not completely. And if something can be remembered, it can come back."
This is such a great little explanation by the Doctor, because it works on two levels. The best is how it is talking about life. People vanish from our lives. Parents die, friends move away. Such is life. As we grow up we too are like the Doctor regenerating: we become someone brand new yet still the same. Sometimes we look back and realize we've forgotten all about someone or something that we loved. We find an old photograph of a long lost friend, or our dead mother's wedding ring...and we remember them. Their presence returns to the room as though they were really in it. Those even more spiritual and sentimental would say that the distant friends can come back, relationships can be reforged, our old dreams from childhood still made true, maybe even the dead loved ones found again in the next life.
When the Doctor tells Amy that people fall out of the world, he is telling the truth. They do.
Now, in the fantasy world of Doctor Who we have a crack swallowing people up randomly.
But the Doctor is already familiar with the premise that was outlined above. Don't get so wrapped up in the idea of being unwritten from time in a precise way. This is not the Star Trek scientific "If you never existed then all of you is gone" concept, but more of a very crude yank out of the universe: like pulling a thread out from a tapestry. When you pull a thread out from a tapestry it is gone, but there's still a trace that it was there. Rory was erased, but he left behind the wedding ring. Amy's parents were erased but they left behind their daughter. You must view this as the tapestry thread, in Doctor Who effect is not always tied to cause: it is all happening at once and only appears linear. Amy can exist after her parents are unwritten, and a wedding ring can be left behind by Rory. Think of it as debris.
A photo can be left behind too, and in Amy's room there was a photo of Rory in a Halloween costume. If Amy looked at the photo she would not have known who Rory was, just like there's tons of photos in your album with people standing next to you that you probably can't remember either.
The nestine took a kind of deep-intense memory scan of Amy. They wanted to bring the Romans to life from deep within her mind, and they did. They also grabbed another Roman, Rory, who was really her boyfriend dressed in costume. They managed to recreate a auton of Rory from deep within Amy where she could still remember what had been taken from her.
Now you must focus on the end of the Doctor's speech: "If something can be remembered, it can be brought back." Much like a passion you forgot you had rekindled, or a friendship lost to time reforged (thank you, Facebook), here we have an extremem idea of memory having great power. Amy pond grew up all those years with a crack in her bedroom. She slept and poured her dreams into the crack, and the crack poured into her. Amy has a kind of fantastic power. She absorbed a part of the crack. As Rory the robot aimed his gun at Amy she somehow, impossibly remembered him. She pulled him back from outside the universe and poured him into the robot body...a few seconds after he was ordered to shoot her. He became both auton and Rory.
When the universe was reset, real Rory was returned. In that moment he had no memoires of his strange stay in the plastic body. When Amy remembered the Doctor, when she found him in the words he left for her, she remembered her adventures with him too. She pulled it all back from outside of the universe, and the memories of Rory as an auton came back too.
This is a human Rory, but inside him is a memory, distant like a dream, of 2000 years as an auton. He can shut the door in his mind and block it out when it gets to be too much for him, but he does remember.
I actually am surprised at how people continue to voice frustration at the bringing back of unwritten things through memory. It makes a lot of sense as long as you try and digest just what the Doctor is saying to Amy in the quote above. You need to also have a bit of a romantic and spiritual view on life.
I never bloody understood 'bad wolf'. Rose left herself a message. AND???? Now THAT makes no sense. Just something that sounds cool.
