linguistic:voice_change, information:filtering, structural:reformatting, quality:corruption
Comedy

One evening in 1912, a performance at the Opera House in Nacogdoches, Texas, was interrupted by shouts from outside about a runaway mule. The audience hurried out to see what was happening. Groucho was angered by the interruption and, when the audience returned, he made snide comments at their expense, including "Nacogdoches is full of roaches" and "the jackass is the flower of Tex-ass". Instead of becoming angry, the audience laughed. The family then realized that it had potential as a comic troupe. (However, in his autobiography Harpo Speaks, Harpo Marx stated that the runaway mule incident occurred in Ada, Oklahoma. A 1930 article in the San Antonio Express newspaper stated that the incident took place in Marshall, Texas.)

The act slowly evolved from singing with comedy to comedy with music. The brothers' sketch "Fun in Hi Skule" featured Groucho as a German-accented teacher presiding over a classroom that included students Harpo, Gummo, and Chico. The last version of the school act was titled Home Again and was written by their uncle Al Shean. The Home Again tour reached Flint, Michigan, in 1915, where 14-year-old Zeppo joined his four brothers for what is believed to be the only time that all five Marx Brothers appeared together on stage. Gummo then left to serve in World War I, reasoning that "anything is better than being an actor!" Zeppo replaced him in their final vaudeville years and in the jump to Broadway, and then to Paramount films.

During World War I, anti-German sentiments were common, and the family tried to conceal its German origin. Upon Minnie Marx learning that farmers were excluded from the draft, she purchased a 27-acre (11 ha) poultry farm near Countryside, Illinois; Stefan Kanfer wrote that "Each night, rats made off with the day’s eggs." During this time, Groucho discontinued his "German" stage personality. In 1917, the Selective Service caught up with Marxes, and each was rejected except Gummo, who was drafted; he spent the war serving in Illinois. Following this, Zeppo (the youngest brother) joined the team.

By this time, "The Four Marx Brothers" had begun to incorporate their unique style of comedy into their act and to develop their characters. Both Groucho's and Harpo's memoirs say that their now-famous on-stage personae were created by Al Shean. Groucho began to wear his trademark greasepaint mustache and to use a stooped walk. Harpo stopped speaking onstage and began to wear a red fright wig and carry a taxi-cab horn. Chico spoke with a fake Italian accent, developed off-stage to deal with neighborhood toughs, while Zeppo adopted the role of the romantic (and "peerlessly cheesy", according to James Agee) straight man.

The on-stage personalities of Groucho, Chico, and Harpo were said to have been based on their actual traits. Zeppo, on the other hand, was considered the funniest brother offstage, despite his straight stage roles. He was the youngest and had grown up watching his brothers, so he could fill in for and imitate any of the others when illness kept them from performing. "He was so good as Captain Spaulding that I would have let him play the part indefinitely, if they had allowed me to smoke in the audience," Groucho recalled. (Zeppo stood in for Groucho in the film version of Animal Crackers. Groucho was unavailable to film the scene in which the Beaugard painting is stolen, so the script was contrived to include a power failure, which allowed Zeppo to play the Spaulding part in near-darkness.) In December 1917, the Marx brothers were noted in an advertisement playing in a musical comedy act "Home Again".

By the 1920s, the Marx Brothers had become one of America's favorite theatrical acts, with their sharp and bizarre sense of humor. They satirized high society and human hypocrisy, and they became famous for their improvisational comedy in free-form scenarios. A famous early instance was when Harpo arranged to chase a fleeing chorus girl across the stage during the middle of a Groucho monolog, to see if Groucho would be thrown off. However, to the audience's delight, Groucho merely reacted by commenting, "First time I ever saw a taxi hail a passenger." When Harpo chased the girl back in the other direction, Groucho calmly checked his watch and ad-libbed, "The 9:20's right on time. You can set your watch by the Lehigh Valley."

The brothers' vaudeville act had made them stars on Broadway under Chico's management and with Groucho's creative direction, with the musical revue I'll Say She Is (1924–1925). Its success helped secure playwright George S. Kaufman and songwriter Irving Berlin —two of Broadway's best talents—for the musical comedy The Cocoanuts (1925–1926) and later Animal Crackers (1928–1929).

Out of their distinctive costumes, the brothers looked alike, even down to their receding hairlines. Zeppo could pass for a younger Groucho and played the role of Groucho's son in Horse Feathers. A scene in Duck Soup finds Groucho, Harpo, and Chico all appearing in the famous greasepaint eyebrows, mustache, and round glasses while wearing nightcaps; the three are indistinguishable, enabling them to carry off the "mirror scene" perfectly.