The original actor might have left, due to injury, contract dispute, illness, or even death. The most common way to handle this is to dispose of the old character in some way, and add a replacement. However, if a producer is too stubborn to write out the original character, they will need to do an actor swap. Two ways to do this are:

• The Other Darrin: Just do it with no explanation.
• The Nth Doctor: Explain it on-screen, using Applied Phlebotinum or Magic Plastic Surgery.

In some cases, an actor may be replaced in the middle of production. Obviously, it would be too confusing to have the same character be played by two different people in the same film/episode. That is, unless your name is Ed Wood. Therefore, this can only be handled by:

• The Other Marty: Reshoot everything with the replacement.

Alternately, the character might need to be aged, or made younger, beyond the capacity of cosmetics to do convincingly. Again, an actor swap is needed. Two common examples are:

• Timeshifted Actor: When the show has hefty time compression, squeezing decades into hours.
• Soap Opera Rapid Aging Syndrome: The show is in real time, but the character ages unnaturally fast.

All these methods dovetail neatly with the Literary Agent Hypothesis, the conceit that to the characters, nothing has changed, because the actors are merely playing roles in stories that "actually happened."

This is the polar opposite of Acting for Two.