Comment by atombender

Stalker has an interesting history, because Tarkovsky did shoot the Strugatskys' own screenplay first.

But almost all of the shot film was accidentally damaged beyond repair by the Soviet lab — they were using specially imported Kodak film stock that apparently the lab was unfamiliar with — and Tarkovsky had to go back to the Soviet film board and negotiate more money to reshoot the film.

Tarkovsky had been unhappy with the film as he shot it, and during these months of downtime, he repeatedly workshopped the script together with the Strugatskys. Long story short, Arkady Strugatsky proposed that Tarkovsky strip down the story; he wrote a treatment that reduced the entire film to a bare-bones, more philosophical story with nameless characters and very few overt sci-fi elements. Tarkovsky essentially wrote everything around that new core, much of it apparently also written during the second shoot.

I recommend the book "The Films of Andrei Tarkovsky: A Visual Fugue", by Johnson and Petrie, which has a whole chapter on Stalker and the difficulties of making that film.

In my opinion, Roadside Picnic is a masterpiece, and I would have loved to see a faithful adaptation of it. Stalker, as it ended up, is not really an adaptation of it.