The year was 1949, and 25-year-old Marlon Brando — “the brilliant brat,” as LIFE magazine called him following his astonishing work on Broadway in A Streetcar Named Desire — had finally answered the call of Hollywood. He was preparing to film his movie debut in The Men, the wrenching story of a World War II vet coping with rage and insecurity after he’s paralyzed in combat. And while it’s true that L.A. was used to next-big-thing newcomers, it was (and still is) exceedingly rare to chronicle the earliest days in the career of a movie actor of Brando’s intensity, eccentricities and electrifying talent.

LIFE photographer Ed Clark captured Brando’s explosive arrival in California, not only trailing the actor as he delved deep into “The Method” — taking to a wheelchair and leg braces to live among paraplegics at a VA hospital in Van Nuys — but also glimpsing more personal sides of Brando, the very private man.

Accompanying Ed Clark’s images in LIFE’s archives were meticulous notes about Brando written by Theodore Strauss, who would ultimately write the magazine’s 1950 profile coinciding with the release of The Men. Strauss details every quirk of the actor: what he wore, how he ate, what he read, how he shunned any sort of red carpet that might have been laid out for him when he came to town.

“Stanley Kramer, producer of The Men, had intended on putting Brando in a good hotel, but Brando would have none of it,” Strauss wrote. “First of all he insisted on living with the paraplegics in Birmingham Veterans Hospital during the four weeks before production began. This, he felt, was necessary to giving a completely knowledgeable and valid performance in his role. At the hospital he was given a bed in a 32-bed ward, where he was treated almost like any other patient.”

The actor’s reputation as a bad boy had preceded him, stories of his nose-picking, shabby dress, foul language and grumbling interviews having traveled all the way from New York to Los Angeles. But, LIFE wrote, “however infantile or irresponsible Brando may be in his personal life, he is a totally conscientious artist in his work. Unlike some of Hollywood’s pretty people, he was never late on the set, never indulged in a tantrum, never required endless retakes.”

He was also far more of an introvert, in some ways, than his reputation suggested. Brando, wrote Strauss, “reads everything, absolutely omnivorous — from Krishnamurti to recent novels.” For the actor, it was all about the craft — nothing else, even life’s essentials, seemed to matter. From LIFE’s profile: “His salary, for the soundest of reasons, has been sent to his father, Marlon Brando Sr., who invests the money in cattle on a Midwestern ranch called Penny Poke. Each week Brando receives a living allowance of $150. Because he rarely looks at money and sometimes pays for a package of cigarets with a $20 bill, he usually is penniless by the second day.”

Of his relationship with the real paraplegics and quadriplegics from whom he was learning for his role, Strauss noted that “Brando’s orientation and adjustment as a paraplegic was so complete that he participated in their sometimes gruesome horseplay with complete freedom — one of the reasons why he was so completely accepted. Monkeyshines include pillow fights, using hypodermic syringes for water pistols.”

“Unless a conference or interview was scheduled at the hospital, Brando slept until noon each day,” Strauss reported in his LIFE piece. “Ordinarily Brando would be rudely awakened when another late-sleeping patient in the next bed slapped his face with a pillow. If Brando woke first, he returned the courtesy.”

From Strauss’ notes about Hollywood’s reaction to Brando: “Thus far no one has accused him of posing; everyone to whom we’ve spoken has a sort of confused respect for a man who, up to now, has managed to live as he feels, without caring a hoot what anyone thinks.”

In his personal style, meanwhile, the actor was unfussy and unpretentious, almost to a fault: “When Brando first arrived in Hollywood his only luggage was a battered, imitation-leather suitcase the size of a woman’s overnight bag,” Strauss observed. “He was wearing a blue worsted suit which had seen much wear and weather — there were holes and tears in the jacket, and a part of Brando was visible through the seat of the pants.”

Finally, hough the movie he was working on would not be a commercial hit, The Men established that Brando was as powerful a presence on film as he was on stage, and silenced those who might have doubted his talent merely because of his eccentricities.

From LIFE: “Even behind his indistinct mutterings of lines, audiences and critics have sensed in Brando a combustible, highly charged spirit, a quicksilver of restlessness of the sort that can command with equal power the empty spaces of a stage or the slick surface of a screen.”

Here, LIFE presents Ed Clark’s photos — many of which were never published in LIFE magazine — of a sui generis film genius at work.