Paul Muni created a sensation in his film debut as a murderer who refuses to divulge his identity or motive in The Valiant and was nominated for the 1928/29 Best Actor Oscar (he repeated the role on television in 1948 for The Chevrolet Tele-Theatre). He continued playing tough guy roles through the early thirties (Scarface, I am a Fugitive From a Chain Gang) before finally settling into a series of stagy though enjoyable performances in prestigious and pretentious screen biographies like The Life of Emile Zola, The Story of Louis Pasteur (Oscar nominated for both and winning for the latter) and Juarez.

Muni's film career petered out in the 1940s and he returned to the Broadway stage where he scored successes in a revival of his star-making role in Counsellor-at-Law, Key Largo, and his greatest triumph Inherit the Wind (for which he won a Tony Award). He received a final Best Actor nomination for his farewell performance in The Last Angry Man, losing to Charlton Heston in Ben Hur.

Other actors to be nominated for the Best Actor Oscar for their film debut were

Laurence Tibbet in The Rogue Song (1929/30)
Orson Welles in Citizen Kane (1941)
Montgomery Clift in The Search* (1948)
James Dean in East of Eden** (1955)
Alan Arkin in The Russians Are Coming, The Russians Are Coming (1966)

*Clift's debut is considered to be The Search because it was his first film that appeared before the public, being released in March of 1948. He actually filmed Red River prior to The Search, but it wasn't released until September of that year.

**Although Dean made an uncredited appearance in Sailor, Beware in 1946, East of Eden is considered his film debut. He shares the distinction with Muni as being the only actors nominated for their first and last movie, receiving a posthumous nomination the following year for his performance in Giant.