go off-book

1. In theater, to rehearse one's lines without having the script in front of one. OK, everyone, you need to practice your lines at home. We want to go off-book tomorrow! I wish you wouldn't ad-lib like that when we go off-book. It throws off the other actors who aren't as confident with their lines. I'm sorry, I'm not ready to go off-book yet—I'm having such a hard time memorizing my lines.

2. By extension, to deviate from the official plan for something. A: "Please tell me she went off-book with that speech." B: "Do you really think I'd write something that incoherent? No, those comments were all her own." You have to watch going off-book when you interview celebrities. If you deviate too much from pre-approved topics, they won't come back on your show. I'm really worried that he's going to go off-book and ramble about nonsense, rather than focus on the teleprompter and deliver the remarks we prepared.