There have been numerous examples of actors who previously played heroes appearing on superhero or sci-fi tv shows and films as other, unrelated, characters.
(This doesn't count actors meeting their later counterparts like Buster Crabbe on
Buck Rogers in the 25th Century, Christopher Reeve and Helen Slater on
Smallville, or Adam West on
Batman the Animated Series!)
The Lone Ranger meets Commando Cody!
Clayton Moore (
The Lone Ranger on
The Lone Ranger tv series) played the lead villainous henchman in
Radar Men from the Moon!
Commando Cody meets Robby the Robot!
George Wallace (
Commando Cody in
Radar Men from the Moon) was the
Bosun in
Forbidden Planet! (Bonus: Richard Anderson [
Oscar Goldman on
Six Million Dollar Man and
Bionic Woman] was the C57-D's
Chief Engineer!)
Spock meets RocketMan / Commando Cody! (sorta)
Leonard Nimoy (
Spock in numerous
Star Trek incarnations) as Narab, a Martian invader in
Zombies of the Stratosphere!
The reason I say "sorta" is that the hero, while named "Larry Martin", wears the
RocketMan /
Commando Cody outfit, and the original script and promo materials list the hero as "Commando Cody", and the supporting characters, carried over from
Radar Men from the Moon, are the same!
PLUS: the actor, Judd Holdren, then played
Commando Cody in a series of new 25-minute films released to both tv and theatres entitled
Commando Cody: Sky Marshal of the Universe!
Commando Cody meets Rocky Jones: Space Ranger!
Richard Crane (
Rocky Jones in
Rocky Jones: Space Ranger) played the sidekick in the
Commando Cody: Sky Marshal of the Universe shorts!
Batman meets Superman!
Robert Lowry (
Batman in the
Batman & Robin serial) appearing as a government agent on
Adventures of Superman!
The Shadow meets The Green Hornet!
Victor Jory (
The Shadow in
The Shadow serial [1940]) playing a villain on
The Green Hornet [1966]!
Dracula meets The Green Hornet!
John Carradine (
Dracula in several films from various studios) playing a demented villian on
The Green Hornet [1966]!
Captain Pike meets The Green Hornet!
Jeffrey Hunter (
Captain Pike the first
Star Trek pilot,
The Cage) as a crooked construction business owner on
The Green Hornet [1966]!
Captain Midnight meets Captain Kirk!
Richard Webb (
Captain Midnight on the
Captain Midnight tv series) as a psycho Starfleet officer on
Star Trek!
The Rocketeer meets Captain Picard!
Bill Campbell (
the Rocketeer in
The Rocketeer) as a lovable rogue on
Star Trek: the Next Generation.
Doc Savage / Tarzan meets Superboy!
Ron Ely (
Doc Savage in
Doc Savage: the Man of Bronze and
Tarzan on
Tarzan [1966-69]) portraying the retired
Golden Age Superman on
Adventures of Superboy!
James Bond meets Superboy!
George Lazenby (
James Bond in
On Her Majesty's Secret Service) as
Jor-El in
Adventures of Superboy.
James Bond meets John Steed (Twice)!
Patrick Macnee (John Steed in
The Avengers and
New Avengers) played
Sir Godfrey Tibbett, an expert on horse breeding consulted by MI-6 killed by the baddies while assisting Bond (Roger Moore).
Macnee also appeared (voice only) in
The Avengers feature film with ex-James Bond Sean Connery as the villain. They had no scenes together.
Macnee also re-teamed with Moore in the World War II film
The Sea Wolves.
John Steed meets Sherlock Holmes (Twice)!
Patrick Macnee also played
Dr. Watson to two different
Sherlock Holmes who had played other genre characters!
Roger Moore (
James Bond /
The Saint) in
Sherlock Holmes in New York
Christopher Lee (
Dracula in numerous Hammer films,
Count Dooku in the
Star Wars series, and
Saruman in the
Lord of the Rings trilogy) in
Sherlock Holmes & the Leading Lady and
Incident at Victoria Falls!
(Lee had also played the villain
Scaramanga opposite Roger Moore as
James Bond in
Man with the Golden Gun!)
John Steed and
James Bond meet the
Man from U.N.C.L.E.!
In the
Return of the Man from U.N.C.L.E.: the Fifteen Years Later Affair tv-movie Patrick Macnee played Sir John Raleigh, the new head of
U.N.C.L.E.!
George Lazenby (
James Bond in
On Her Majesty's Secret Service) cameoed in the tv-film as
"JB", driving an Aston-Martin DB-5 and saving
Napoleon Solo with the car's weaponry!
John Steed and Napoleon Solo meet Kwai Chang Caine!
Both Robert Vaughn (
Napoleon Solo on
The Man from U.N.C.L.E.) and Patrick Macnee (
John Steed) appeared several times on
Kung Fu: the Legend Continues.
Their characters are implied to be
Solo and
Steed whose identities have been changed when they retired.
Barnabas Collins meets Spock!
Ben Cross (
Barnabas Collins in the 1990s revival of
Dark Shadows) played
Sarek in
Star Trek [2009]!
While most of his scenes are with Zachary Quinto as
Spock, there's also a deleted scene on the DVD of Cross and Nimoy (as
Spock)!
There have even been meetings of two actors who played heroes, and
neither was a heroic character!
Adam West (
Batman on
Batman [1966]) met Alec Baldwin (
The Shadow in
The Shadow [1995]) on
30 Rock!
Know any others?