There are several reasons why Humphrey Bogart became a star. Among them:
1) Leslie Howard's loyal friendship. Had Howard not insisted that Warner Brothers hire Bogart to reprise his gangster role in the film version of Howard's stage hit The Petrified Forest, Bogart may have never made it in the movies.
2) A trio of hit films from 1940 to 1942: Although audiences liked Bogart during his supporting role years, it wasn't until HIGH SIERRA, THE MALTESE FALCON and CASABLANCA that Bogey became a first-ranked star.
3) George Raft's bad career choices. Raft turned down the lead roles in HIGH SIERRA and THE MALTESE FALCON, clearing the path for Bogart's breakout performances.
4) Talent. The man had tons of it.
5) Perseverence. Once Bogart signed with Warners, he played any part they asked of him. He grumbled, he bitched, he complained, but he kept working, putting his face before movies audiences several times a year. Unlike fellow actors James Cagney or Bette Davis, he didn't fight for his career by refusing parts, he fought for his career by constantly working. He described himself as the guy who complained about parts but never turned one down. "I've never forgotten a piece of advice [actor] Holbrook Blinn gave me," Bogart once said. "'Just keep working'. The idea is if you're always busy, sometime somebody is going to get the idea that you must be good."
This last point is most important when viewing THE RETURN OF DR. X, in which Bogie plays a bespectacled rabbit-petting undead doctor who must remain 'alive" by draining the blood of other humans. Just keep working. - JB