The film score...
....I know was by Miklós Rózsa, but I'd love to know who played the handsaw.
Not a handsaw, it was something called a Theramin, some kind of early electronic musical device. Rosza first used it in the score for Spellbound and Selznick was thrilled to have the first Hollywood score to use the theremin.
However, Spellbound had a difficult and lengthy post production period due to a dream sequence designed by Salvador Dali and it delayed the release of Spellbound by many months. Meanwhile Rosza was hired to do the score for The Lost Weekend and suggested the theremin be used for the score and it worked great for the crazed elements of that film.
Weekend was a simpler film to produce and was actually released before Spellbound being the first film to expose this unique sound to audiences and Selznick was not happy with this. But it sure wasn't Rosza's fault Spellbound was released late.
Both are good films with great soundtrack scores.