There was that tightening of his expression again as if a dark cloud had crossed over his soul. River wanted so badly to know what he was keeping from her, but she was also very afraid of it. It would be her undoing, she was sure. She always knew that the Doctor would be her undoing, one way or another. She told her father once, a very long time ago in 1960s America, that someday she would look upon this handsome, brilliant man, and he wouldn't know her. It was going to kill her. She knew that day was looming close, but she didn't know just how close it was. Thankfully, this incarnation of him seemed to know, trust her even.
"How do you kill the Doctor?" she said weakly, gripping his hand tightly in hers Why, she knew a variety of ways. A simple kiss with her lips tainted with the poison of the Judas Tree. Shooting him once, and shooting him again during his regeneration. She was raised to know how to kill him. Now that she thought about it, it seemed only fair that in the end, he would be her undoing. No matter how much he forgave her for her sins against him, it was hard for her to do so even now. A part of her almost felt like she deserved what darkness that was waiting in store for her.
Swallowing the tightness in her throat, she kept her focus. The Doctor was on a roll with a theory, and he would be rolling with it. She needed roll on with him or else be left behind "Someone would have to know your weaknesses. Know you. They'd have to study you from every angle and use any information they could get about you against you," she answered in a strained tone. Hadn't Madame Kovarian done exactly that? River reminded herself that was a long time ago, and the Doctor had freed her from Kovarian. Now she was free from prison. She had to keep moving towards the future, her future.