Lahr's most famous role was that of the Cowardly Lion in Metro-Goldwyn-Mayer's 1939 adaptation of The Wizard of Oz. Lahr was signed to play the role on July 25, 1938. He starred opposite Judy Garland, Ray Bolger, Jack Haley, Frank Morgan, and Margaret Hamilton. Lahr's lion costume was composed of lion fur and, under the high-intensity lighting required for Oz's Technicolor scenes, the costume was unbearably hot. Lahr contributed ad-lib comedic lines for his character. Many of Lahr's scenes took several takes because other cast members, especially Garland, couldn't complete the scenes without laughing. The Cowardly Lion is the only character who sings two solo song numbers-"If I Only Had the Nerve", performed after the initial meeting with Dorothy, The Scarecrow, and The Tin Man in the forest, and "If I Were King of the Forest", performed while he and the others are awaiting their audience with the Wizard.