What term is used to inform actors that the show will begin in thirty minutes?

The term used to inform actors that the show will begin in thirty minutes is "Half." This call acts as a significant cue in the theater production process, letting performers know they need to prepare to go on stage soon.

The term "Half" serves as a reliable marker in the schedule, allowing actors to finish their last-minute preparations, warm up, and get into character. It helps to create a structured timeline leading up to the performance, ensuring that everyone is aware of the approaching curtain time.

The other terms presented in the options refer to different aspects of theater and performance. "Interval" refers to a break in the performance, "Preview" is associated with showing a production before its official opening for potential audiences, and "On the Book" is a phrase used to indicate that an actor is performing with a script available rather than fully memorized. Each of these terms serves a unique role within theater, but "Half" specifically indicates the time remaining before the performance begins.