Howard Duff made an uncredited appearance as Sam Spade (albeit without mentioning the character's last name) in the Dec. 5, 1946 episode of Suspsense, "The House in Cypress Canyon." Both Suspense and Sam Spade were produced by William Spier, who arranged for Duff's cameo.
Howard Duff: (on auditioning for Sam Spade on radio) There must have been at least 100 other guys jammed in that office waiting to read. I even recognized a few famous faces in the crowd, and it threw me into even a greater melancholy. By the time my turn came I was feeling real mean and about as low as a patrolman's instep. When they handed me the script and told me to go ahead, I delivered the lines in a half-snarling, half-bored manner like a guy reading a grocery list.