Walk-ons and Tag-outs are two of the most important weapons in an improviser's arsenal when doing a longform improv show. And much like any weapon, they are deadly in the hands of someone skilled and dangerous in the hands of someone unskilled. This post isn't to suggest that I am skilled, but rather that I have a philosophy about what these weapons are and how they should be used to their most effectiveness to kill the audience and not the scene.
A scene is initiated by two people. Each of these people brings with them a deal. Suppose person A's deal is YELLOW and person B's deal is BLUE. This means that the scene they are doing should be GREEN.
If you do a walk-on (or phone call, or backline noise, etc.) into this scene your objective should be to make this scene GREENER.
If you do a tag-out and tag out person A, leaving person B, then your objective should be to make this scene BLUER. If you do a tag-out and tag out person B, leaving person A, then your objective should be to make this scene YELLOWER.
The walk-on is a way to highlight and heighten something happening in the scene. A tag-out is a way to highlight and heighten something happening with a character.
So many people wield this weapon poorly and people get hurt.