Carol Reed had left Two Cities Films and Rank in early 1947 after a quarrel over the budget and final edit of Odd Man Out. He joined Korda's new London Films set-up at Shepperton, where (bolstered by a loan from the National Film Finance Corporation, and a distribution deal with 20th Century– Fox) accountants had lower priority, and the atmosphere was more creative. An invitation to score Reed's first Korda production, The Fallen Idol, as well as Anthony Asquith's The Winslow Boy (1949), meant Alwyn now worked without his usual ally Mathieson, who was under contract to Rank. At London Films, the music director was Hubert Clifford, recently appointed in late 1946. Clifford had written his first score in 1943, for a documentary, and like Alwyn was a professor of composition at the RAM . By the time of The Fallen Idol he was fresh from a trip to Hollywood to study film music and sound recording techniques. Clifford had strong views about the subject of leitmotif, believing it made little impression on audiences. Leitmotif is of no great significance to either The Fallen Idol or The Winslow Boy.

It was Korda's inspiration to join the creative talents of Graham Greene and Carol Reed. Together they constructed from Greene's pre-war short story “The Basement Room” (1935) a screenplay about three days in the life of young Phillipe or “Phil” (Bobby Henrey), an eight-year-old ambassador's son living in a large mansion in Belgrave Square. A major part of Reed's achievement was to draw from Henrey a memorably unselfconscious performance of a child who, deprived of parental affection, worships the butler Baines as surrogate father. As the kind, self-sacrificing Baines, Ralph Richardson's performance rings true : he is trapped between his love for Julie (Michele Morgan), a young secretary at the embassy, and his marriage to a wife he dislikes (Sonia Dresdel). Dresdel gives a truly cankerous performance as Mrs Baines, whose suspicions turn her from an unfeeling, cold woman into a desperately spiteful one. Left alone with Baines and Julie for a weekend, Phil misunderstands the repressed adult relationships. When Mrs Baines is accidentally killed, Phil believes Baines has murdered her and lies to save him from arrest.

Themes are interlinked, and lie-telling structures the plot.