Dino Walked Off This Movie (SPOILERS)
When "Showdown" was made in 1972 for 1973 release, Dean Martin was very, very rich, but a bit past his prime as a top movie star. He'd been THE top TV variety show star since 1965, doing movies on the side (mainly Westerns, plus those silly Matt Helm spy movies), while packing them in in Vegas and generally making tons of dough off of his creative pursuits and land buying.
But the 70's were underway and things were changing. Dino's TV show ratings were declining. He was looking older. And he just wasn't a top movie star anymore. Neither was pallie Frank Sinatra, who made a comedy Western called "Dirty Dingus Magee" in 1971 and pretty much walked away from movies for good (less a "comeback movie or two," like "The First Deadly Sin.")
Dean Martin signed up for "Showdown" opposite Rock Hudson, and the two men certainly brought their charisma and likeablity to this minor Western. They played boyhood friends grown up to wrong sides of the law: Hudson a lawman, Martin a non-violent train robber. Dean brought his laidback TV persona to his outlaw role and made that a charming vulnerability: people want to HANG good ol' Dino; nastier outlaws want to kill him. Hey, he just wants a martini. Hudson tries to keep him alive.
Amiable as Dean Martin looks in "Showdown," something went wrong. His biographer, Nick Tosches, suggests that the older, very rich Martin knew his career was starting its downturn (in movies and TV only; he'd always be in demand in Vegas) and was not happy packing a toy cap pistol and playing cowboy in his fifties. He didn't need this anymore. He had plenty of cash.
So Dean Martin walked off of the "Showdown" location and quit the movie.
It turned into quite the newspaper story, with Universal chief Lew Wasserman threatening Martin with a lawsuit and people gossiping about a "meltdown" in Dino's life. But it seems, in retrospect, that Martin was just beginning the rest of his life with a vengeance: he just didn't care anymore.
I'm not sure how the "Dino walkout" ended, but "Showdown" as we have it suggests that he walked out on the climactic gunbattle sequence. For we never actually SEE what happens to Martin in that gunbattle. His friend Hudson ends up with Martin's conveniently wrapped-up corpse to bury.
This would suggest that Martin indeed never came back to "Showdown." He probably figured there was enough of him on film to cobble together the ending scenes without him. He was sort of right.
Dino did one more movie lead ("Mr. Ricco" in 1975, that nobody saw), and quit being a movie star.