Throughout the history of commercial arts, creators are limited by what's available and what they can afford. Sometimes the artist will be forced to go another direction. Many do it gracefully, others leave so much of their original idea in the piece that what remains can only be seen as a poor substitute.
- Star Trek: Generations's producers hoped to get DeForest Kelley to appear as Dr. McCoy, but due to his declining health, he could not get on-set insurance. Most of his lines in the opening scene were given to Pavel Chekov, which is why Chekov drafts reporters as nurses and heads to Sickbay. Scotty was given one line, asking Kirk if he wanted a tranquilizer. (see here)
- Likewise, they wanted Leonard Nimoy to play Spock, but when he declined his lines were given to Scotty.
- A music variant occurs in the final scenes of The Devil's Advocate. Evidently the filmmakers wanted to end the film with "Sympathy For The Devil" By The Rolling Stones. It would seem they had to settle for less appropriate "Paint It Black".
- A gender switch variant in Ballistic: Ecks vs. Sever. The part of Sever was originally supposed to be male, but after Lucy Liu was cast in the role the script wasn't changed to reflect this. That's why there's no UST between her and Ecks (Antonio Banderas) the way you'd expect. Source: IMDb Trivia.
- In a college Q&A, Kevin Smith discussed his experience with this. He wanted to use "The Most Beautiful Girl In The World" by Prince in Jay and Silent Bob Strike Back when Jay first sees Justice. Prince refused and offered him "Raspberry Beret" instead. When Smith tried to explain why that just didn't fit the scene, the conversation went nowhere, and he chose to go with "Bad Medicine" by Bon Jovi instead.
- Halle Berry's role in The Flintstones was originally meant to be played by Sharon Stone. What makes this obvious? The character's name is Sharon Stone.
- Get Smart. Don Adams was unavailable for the episode "Ice Station Sigfried" so Maxwell Smart's place was taken by a CIA agent named Quigly, played by Adams' friend Bill Dana. You can tell that very little rewriting was done to the script, and the writers simply gave Maxwell Smart's lines to Quigly.
- This happened a lot in classic Doctor Who.
- The most notable example is "The Five Doctors", which Terrance Dicks wrote with actor availability in mind. Here is the problem: "The Five Doctors"' impressive cast forced Dicks to write those characters as generically as possible, and it's glaringly obvious in many places.
- "Mawdryn Undead" was originally written as a return appearance by Ian Chesterton, but the actor was unavailable. It was rewritten for Brigadier Lethbridge-Stewart, but the school setting was retained, with a handwave about the Brig retiring from the military and becoming a teacher.
- Lampshaded in The Cinema Snob's review of the A Clockwork Orange porn parody A Clockwork Orgy, which replaces "Singing In The Rain" with "Take Me Out To The Ballgame":
Snob: Oh yeah, Gene Kelly's second most popular song! Let me guess, they're going to replace Beethoven with "On Top of Spaghetti"!
- In The Nostalgia Critic's review of Garfield: A Tale of Two Kitties, he believes the character Lord Dargis, played by Billy Connolly in the film, was obviously written with John Cleese in mind.