Possible Clues:

• It's implicit in knock-knock jokes
• Actor given space to find an exit
• Something most people go through at some point
• "Let's Make a Deal" offering
• Shut it up or cross it up
• One heckling opener faced Marley here?
• Word with ''step'' or ''stop''
• Opener cross when shown up
• Monty Hall offering
• There are frequent openings here