In a script, an "in" is typically indicated with the word "INT." or "EXT.," followed by the location and a brief description of the scene. For example, "INT. LIVING ROOM - NIGHT." would indicate the start of a new scene taking place inside a living room at night.
The term "in" is often used in conjunction with "out," which refers to the end of a scene or shot, or the point at which a character exits the scene. "Out" is typically indicated with the word "OUT," followed by the location and a brief description of the scene. For example, "OUT. LIVING ROOM - NIGHT." would indicate the end of a scene taking place inside a living room at night.
"In" and "Out" are important terms for understanding the structure of a film or TV script, and for communicating with other crew members about the sequence of shots.