Possible Clues:

• Imagined barrier between actor and audience
• Invisible impediment in the theater
• Imaginary theater boundary
• What a character breaks when they talk directly to the audience
• Something a character might break
• Ferris Bueller breaks it
• Thing broken by Deadpool (after 13-Down, 22-Down and 51-Down)
• Dramatic split worth a full apology?
• It separates actors from audiences