I Created a simplified example
When you enters the trigger box, it SETs the “CanTurnsOn” varibale to TRUE and show the message: “Press “G” to Play Matinee”
If the player leaves the trigger box, it SETs the variable to FALSE
This way, it will only plays the Matinee if the player is inside the trigger box, if not, the G key will have no effect!
See the image BP:
Hope it helps you!