When was the first occurrence of an actor reprising his role as the same character in a totally unrelated show or movie?
Ursula Buffay from Friends appearing on Mad About You would be a good example, or Ray Stantz from Ghostbusters appearing in Casper.
Crossovers within the same franchise (such as the Arrowverse) and spin-off shows (like Joanie Loves Chachi) are not counted here. The shows must be otherwise unrelated.
Richard Belzer, a former stand-up comedian, played the character of Detective John Munch.
The character of John Munch, as portrayed by Belzer, began in 1993 on NBC's Homicide: Life on the Street, set in Baltimore.
In 1999, Homicide had a crossover with Law & Order. Later in that same year, Munch then joined Law & Order: Special Victims, as a regular character.
In the end, Belzer, as Munch, appeared on NBC's Homicide, Law & Order, Law & Order: SVU, Law & Order: Trial by Jury and 30 Rock. He also appeared (again, as Munch) in Fox's Arrested Development and The X-Files; UPN's The Beat; HBO's The Wire; NetFlix's Unbreakable Kimmy Schmidt; and finally, on ABC's Jimmy Kimmel Live.
Charlie Chaplin played the Tramp in dozens of films. I don't know what you mean by "unrelated", but maybe some pairs of them qualify. The first three were released in February 1914.
Probably also some earlier films shared characters among each other.
Peter O'Toole played King Henry II in Becket (1964) and The Lion in Winter (1968).
