Well, I swear I must have seen something like this in a TV show (probably not a movie). From this thread it sounds like it wasn’t the original source, but instead someone referencing the original.
I’m picturing someone like John Malkovich or Adrian Monk. But who knows. (and that’s the beauty of it. 
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Adams started his career using paper and ink. I can see how he might have worked that way. I know that when I write something lengthy and then reread it, the flaws in the composition become evident, and a rewrite looks called for. Doing it page by page would be less writing and going back through the whole thing over again.
Being able to edit in a word processor obscures this process quite a bit.
There’s THROW MOMMA FROM THE TRAIN, where Billy Crystal has been (a) resisting Danny DeVito’s attempts to rope him into a Strangers-On-A-Train plan to murder each other’s target, and (b) struggling for the better part of a year to craft a single perfect sentence — which, of course, prompts DeVito’s intended victim to supply Crystal with the ideal wording as if it’s the easiest thing in the world.
Did Snoopy ever get past “It was a dark and stormy night.” when he wrote?
Of course he did. They even put out a book with the whole text, and Snoopy’s preferred cover (it’s inside this book, though):
Here it is:
I can even remember some of Snoopy’s text: “The maid screamed. A door slammed. Suddenly a pirate ship appeared on the horizon!”
As Snoopy thinks, “This twist in the plot will baffle my readers.”
Ya, Tidewater Tales. It’s been too long, but I believe it was a writer’s block problem that afflicted one of the main characters, Peter. Probably not who the OP is thinking of, as Peter had written at least one other novel before. Dang I like Barth. Couldn’t speak right for a week after I finished Sotweed Factor.
Not exactly fictional, but Joe Gould’s Secret, by Joseph Mitchell, was an account of a barfly / street character of that name who spent years lugging around notebooks containing his Oral History of New York, which he insisted would be published one day and reveal a side of the city and its people seldom revealed. But he didn’t allow anyone to see any of it, and though he impressed many of his acquaintances with his intelligence and wide-ranging knowledge of the city’s history, most doubted the book existed or would ever see the light of day.
Some time after he died, his stash of notes was discovered, and mainly it consisted of multiple rewrites of a few stories from his early years - he seems to have never been happy with it, and never got very far.
Sorta kinda writer’s block. He’d decided he needed a more minimalist approach, also known as “less is more”, and his books had gone from fairly sprawling to a word or two on a page. His latest work in progress was completely blank. It wasn’t so much that he couldn’t write but that he kept editing his writing down to nothing in his quest to convey the most meaning and implication and atmosphere and all the other things in as few words as were absolutely necessary, and as time went on he found fewer words to be absolutely necessary.