As the old joke says: very carefully.

It's a complicated system of scales and percentages governed by a 400-page contract. But basically, when actors sign on to appear in TV shows, they're paid a fee that covers the first airing of the show. When that show is used again, whether as a rerun, in a DVD boxed set or on a streaming website, the actors are paid what's called a residual.

The minimum amount of the residual is set out by the collective agreement between the actors union (SAG-AFTRA -- the Screen Actors Guild/American Federation of Television and Radio Artists) and the Alliance of Motion Picture and Television Producers (AMPTP), an association that negotiates on behalf of TV and film production companies. The details vary for major stars, but the basic formula is that actors are paid a declining percentage of their original paycheck each time an episode is rerun until the 13th time it airs -- at that point, actors earn five per cent of their original pay, and it stays at that level.

As you can imagine (especially if you think about your "Little House on the Prairie" example), this results in a lot of paychecks. SAG-AFTRA says it mails out millions of residuals checks every year. 

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