There's a fascinating interview first published in the Cahiers des amis de Roger Nimier 6 (1989) in which Marc Dambre interviews Louis Malle, who talks about Ascenseur pour un échafaud. After working as Robert Bresson's assistant he wrote a script which producers weren't interested in. Then his friend Alain Cavalier introduced him to Noël Calef's book: the idea of a person accused of murder in his absence appealed to Malle, who then decided that he wanted to work on a script with Roger Nimier as he liked his novels. This was to be Malle's first film as a director.
The part of Jeanne Moreau as Florence Carala doesn't exist in the book. Then, she had been making a name for herself as a theatre actor and had played in film noirs with Jean Gabin, but it was with Ascenseur that she began to be really recognised. Malle originally saw the film as an 'exercise de style', but thirty years later he saw himself then as an ambitious person trying to come to terms with his love of both Bresson and Hitchcock. Nimier hated the novel, but they continued re-working it until they returned to Julien Tavernier (Maurice Ronet) stuck in the lift of the Carala building after killing Simon Carala (Jean Wall), with the young Louis (Georges Poujouly) killing the German Benckers as his girlfriend Véronique (Yori Bertin) looks on, and Moreau being added to flesh out the story.
Malle would work on the script with Nimier in the evening until midnight and then they'd go and join Antoine Blondin, although he adds that he didn't like getting in a car with Nimier! He describes his former self and friends as 'Happy, pessimistic "dandies", mocking the Bourgeois-Stalinist intelligentsia of the period.' (The interview was translated by Hugo Frey.)
Another interesting fact is that, after watching the film twice, Miles Davis wrote the music – the mournful wailing trumpet – one night between ten in the evening and six in the morning. A number of people speak of how much the music enhances the film, although I'm not too sure: I think it's way too loud.