For the ages, really.
Q: How did Gene Wilder come to play the Waco Kid in Blazing Saddles?
A: Wilder had been the #2 choice in casting. He was hastily called in on the second day of filming to replace ...wait for it... Gig Young, who'd been having DTs on the set on the first day.
(A Western movie binge? Nah: after seeing the 2007 3:10 to Yuma I ordered the 1957 version to refresh my memory, heard Frankie Laine sing its title theme, went to the Web to verify that he'd done numerous other songs for Westerns including Blazing Saddles, followed another link for the latter, and bingo. See how much you can learn when you no longer trust your memory?)