It’s a huge gamble on Cersei’s part. You need to be steely and tough as hell to pull a move like this on a person like Tywin. She goes up against him and she wins. She wins with the one thing that she holds over him, which is his family legacy. She has a piece of information that could destroy the most important thing in the world to him, the thing he’s worked on his entire life. And I think that Tywin looks in her eyes and when she tells him the truth he realizes that she is someone who will do exactly what she claims she’ll do. She’s very much her father’s daughter in that way.
(D.B. Weiss)