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1944–1950: Early career

Brando used his Stanislavski System skills for his first summer stock roles in Sayville, New York, on Long Island. Brando established a pattern of erratic, insubordinate behavior in the few shows he had been in. His behavior had him kicked out of the cast of the New School's production in Sayville, but he was soon afterwards discovered in a locally produced play there. Then, in 1944, he made it to Broadway in the bittersweet drama I Remember Mama, playing the son of Mady Christians. The Lunts wanted Brando to play the role of Alfred Lunt 's son in O Mistress Mine, and Lunt even coached him for the audition, but Brando made no attempt to even read his lines at the audition and was not hired. New York Drama Critics voted him "Most Promising Young Actor" for his role as an anguished veteran in Truckline Café, although the play was a commercial failure. In 1946, he appeared on Broadway as the young hero in the political drama A Flag is Born, refusing to accept wages above the Actors' Equity rate. In that same year, Brando played the role of Marchbanks alongside Katharine Cornell in her production's revival of Candida, one of her signature roles. Cornell also cast him as the Messenger in her production of Jean Anouilh 's Antigone that same year. He was also offered the opportunity to portray one of the principal characters in the Broadway premiere of Eugene O'Neill 's The Iceman Cometh, but turned the part down after falling asleep while trying to read the massive script and pronouncing the play "ineptly written and poorly constructed".

In 1945, Brando's agent recommended he take a co-starring role in The Eagle Has Two Heads with Tallulah Bankhead, produced by Jack Wilson. Bankhead had turned down the role of Blanche Dubois in A Streetcar Named Desire, which Williams had written for her, to tour the play for the 1946–1947 season. Bankhead recognized Brando's potential, despite her disdain (which most Broadway veterans shared) for method acting, and agreed to hire him even though he auditioned poorly. The two clashed greatly during the pre-Broadway tour, with Bankhead reminding Brando of his mother, being her age and also having a drinking problem. Wilson was largely tolerant of Brando's behavior, but he reached his limit when Brando mumbled through a dress rehearsal shortly before the November 28, 1946, opening. "I don't care what your grandmother did," Wilson exclaimed, "and that Method stuff, I want to know what you're going to do!" Brando in turn raised his voice, and acted with great power and passion. "It was marvelous," a cast member recalled. "Everybody hugged him and kissed him. He came ambling offstage and said to me, 'They don't think you can act unless you can yell.'"

Critics were not as kind, however. A review of Brando's performance in the opening assessed that Brando was "still building his character, but at present fails to impress." One Boston critic remarked of Brando's prolonged death scene, "Brando looked like a car in midtown Manhattan searching for a parking space." He received better reviews at subsequent tour stops, but what his colleagues recalled was only occasional indications of the talent he would later demonstrate. "There were a few times when he was really magnificent," Bankhead admitted to an interviewer in 1962. "He was a great young actor when he wanted to be, but most of the time I couldn't even hear him on the stage."

Brando displayed his apathy for the production by demonstrating some shocking onstage manners. He "tried everything in the world to ruin it for her," Bankhead's stage manager claimed. "He nearly drove her crazy: scratching his crotch, picking his nose, doing anything." After several weeks on the road, they reached Boston, by which time Bankhead was ready to dismiss him. This proved to be one of the greatest blessings of his career, as it freed him up to play the role of Stanley Kowalski in Tennessee Williams ' 1947 play A Streetcar Named Desire, directed by Elia Kazan. Moreover, to that end, Bankhead herself, in her letter declining Williams' invitation to play the role of Blanche, gave Brando this ringing—albeit acid-tongued—endorsement stating "I do have one suggestion for casting. I know of an actor who can appear as this brutish Stanley Kowalski character. I mean, a total pig of a man without sensitivity or grace of any kind. Marlon Brando would be perfect as Stanley. I have just fired the cad from my play, The Eagle Has Two Heads, and I know for a fact that he is looking for work".

Pierpont writes that John Garfield was first choice for the role, but "made impossible demands." It was Kazan's decision to fall back on the far less experienced (and technically too young for the role) Brando. In a letter dated August 29, 1947, Williams confided to his agent Audrey Wood: "It had not occurred to me before what an excellent value would come through casting a very young actor in this part. It humanizes the character of Stanley in that it becomes the brutality and callousness of youth rather than a vicious old man... A new value came out of Brando's reading which was by far the best reading I have ever heard." Brando based his portrayal of Kowalski on the boxer Rocky Graziano, whom he had studied at a local gymnasium. Graziano did not know who Brando was, but attended the production with tickets provided by the young man. He said, "The curtain went up and on the stage is that son of a bitch from the gym, and he's playing me."

In 1947, Brando performed a screen test for an early Warner Brothers script for the novel Rebel Without a Cause (1944), which bore no relation to the film eventually produced in 1955. The screen test is included as an extra in the 2006 DVD release of A Streetcar Named Desire. Brando's first screen role was a bitter paraplegic veteran in The Men (1950). He spent a month in bed at the Birmingham Army Hospital in Van Nuys to prepare for the role. The New York Times reviewer Bosley Crowther wrote that Brando as Ken "is so vividly real, dynamic and sensitive that his illusion is complete" and noted, "Out of stiff and frozen silences he can lash into a passionate rage with the tearful and flailing frenzy of a taut cable suddenly cut."

By Brando's own account, it may have been because of this film that his draft status was changed from 4-F to 1-A. He had had surgery on his trick knee, and it was no longer physically debilitating enough to incur exclusion from the draft. When Brando reported to the induction center, he answered a questionnaire by saying his race was "human", his color was "Seasonal-oyster white to beige", and he told an Army doctor that he was psychoneurotic. When the draft board referred him to a psychiatrist, Brando explained that he had been expelled from military school and had severe problems with authority. Coincidentally, the psychiatrist knew a doctor friend of Brando. Brando avoided military service during the Korean War.

Early in his career, Brando began using cue cards instead of memorizing his lines. Despite the objections of several of the film directors he worked with, Brando felt that this helped bring realism and spontaneity to his performances. He felt otherwise he would appear to be reciting a writer's speech. In the TV documentary The Making of Superman: The Movie, Brando explained: "If you don't know what the words are but you have a general idea of what they are, then you look at the cue card and it gives you the feeling to the viewer, hopefully, that the person is really searching for what he is going to say—that he doesn't know what to say". Some, however, thought Brando used the cards out of laziness or an inability to memorize his lines. Once, on the set of The Godfather, Brando was asked why he wanted his lines printed out. He responded: "Because I can read them that way."