The scene where Borden tells Olivia just how easily he was able to see through Angier's act:
Olivia: He wants me to come work for you and steal your secrets.
Borden: What does he need my secrets for? His trick is top-notch. He vanishes, and then he reappears instantly on the other side of the stage - mute, overweight, and unless I'm mistaken, very drunk. It's astonishing, how does he do it?
And before that when Angier first sees Root on stage:
Angier: Great! Now all I have to do is keep my self stinking drunk and no one will be able to tell the difference