Though a straight man on stage, he was reputed to be very funny in person, perhaps the funniest of his siblings. As the youngest and having grown up watching his brothers, he could fill in for and imitate any of the others when illness kept them from performing. "He was so good as Captain Spaulding [in Animal Crackers] that I would have let him play the part indefinitely, if they had allowed me to smoke in the audience," Groucho recalled. (Zeppo did impersonate Groucho in the film version of Animal Crackers in the blackout scene in which the Beaugard painting is stolen. Despite popular belief, the blackout scene was not added to the film because Groucho fell ill, but was in fact in the original Broadway play.) However, he never invented a comic persona of his own that could stand up against those of his brothers. As critic Percy Hammond wrote, sympathetically, in 1928,
"One of the handicaps to the thorough enjoyment of the Marx Brothers in their merry escapades is the plight of poor Zeppo Marx. While Groucho, Harpo and Chico are hogging the show, as the phrase has it, their brother hides in an insignificant role, peeping out now and then to listen to plaudits in which he has no share."
Though Zeppo continued to play straight in the Brothers' movies at Paramount, he did occasionally get to be part of classic comedy moments in them--in particular, his role taking dictation from Groucho in Animal Crackers. As he also showed in the movies, he probably had the best singing voice of the brothers.
The popular assumption that his character was superfluous was fueled in part by, interestingly enough, Groucho. According to Groucho's own story, when the group became the Three Marx Brothers, the studio wanted to trim their collective salary, and Groucho replied, "We're twice as funny without Zeppo!"
Offstage, Zeppo had great mechanical skills and was largely responsible for keeping the Marx family car running. Zeppo later owned a company which machined parts for the war effort during World War II, Marman Products Co. Inglewood, CA later known as the Aeroquip Co. (still in business today) This company produced a motorcycle, called the Marman Twin and the Marman clamps used to hold the "Fat Man" atomic bomb inside the Enola Gay He also founded a large theatrical agency with his brother Gummo, and invented a wristwatch that would monitor the pulse rate of cardiac patients and give off an alarm if they went into cardiac arrest.
During his time as a theatrical agent, he and Gummo, although primarily Gummo, represented their brothers, among many others.
The last surviving Marx Brother, Zeppo died of lung cancer in 1979 at the age of 78.
James Agee considered Zeppo "a peerlessly cheesy improvement on the traditional straight man. Along similar lines, Gerald Mast, in his book The Comic Mind: Comedy and Movies (University of Chicago Press: 1979), notes that Zeppo's comedic persona, while certainly more subtle than his brothers', is undeniably present:
[He] added a fourth dimension as the cliché of the [romantic] juvenile, the bland wooden espouser of sentiments that seem to exist only in the world of the sound stage. [... He is] too schleppy, too nasal, and too wooden to be taken seriously. (282, 285).
Danél Griffin, film critic for the University of Alaska Southeast, elaborates on Mast's theory:
Zeppo's parts were always intended to be a parody of the juvenile role often found in sappy musicals of the 1920s-30s era. Sometimes, he would just have a few lines, and he would otherwise be reduced to standing in the background with a big smile on his face. In these roles, he was a lampoon of the infamous extra, always grinning widely as a needless decoration, and always stiff and wooden. In other films, Zeppo would have a more significant role as the romantic lead, but he would still always be stiff, wooden, and, yes, with a big smile on his face. Either way, he could never be considered a real straight man. He was a sappy distortion of the real thing, and sort of the gateway through which we connected with the other Brothers. We perceived him as the "normal, good-looking" one of the bunch, but was he really? Wasn't there something about that line from The Cocoanuts, 'You can depend upon me, Mr. Hammer,' that was a little too ... happy? Roger Ebert called Zeppo 'superfluous,' and that is the point of his character in the five Paramount films. He was the straight man only in pure Marxian sense — while his Brothers spat on movie clichés, he imitated them, proving in his own way to be quite a brilliant comedian.In her book Hello, I Must be Going: Groucho & His Friends, Charlotte Chandler defends Zeppo as being "the Marx Brothers' interpreter in the worlds they invade. He is neither totally a straight man nor totally a comedian, but combines elements of both, as did Margaret Dumont. Zeppo's importance to the Marx Brothers' initial success was as a Marx Brother who could 'pass' as a normal person. None of Zeppo's replacements (Allan Jones, Kenny Baker, and others) could assume this character as convincingly as Zeppo, because they were actors, and Zeppo was the real thing, cast to type" (562).
Zeppo's comic persona is highlighted in the "letter scene" of Animal Crackers. In his book Groucho, Harpo, Chico, and Sometimes Zeppo, Joe Adamson analyzes the scene, showing how it reveals Zeppo's ability to one-up Groucho with simple, plain-English rebuttals. In the scene, Zeppo is told to take a letter to Groucho's lawyer. Adamson notes,
There is a common assumption that Zeppo = Zero, which this scene does its best to contradict. Groucho dictating a letter to anybody else would hardly be cause for rejoicing. We have to believe that someone will be there to accept all his absurdities and even respond somewhat in kind before things can progress free from conflict into this genial mishmash. Groucho clears his throat in the midst of his dictation, and Zeppo asks him if he wants that in the letter. Groucho says, 'No, put it in the envelope.' Zeppo nods. And only Zeppo could even try such a thing as taking down the heading and the salutation and leaving out the letter because it didn't sound important to him. It takes a Marx Brother to pull something like that on a Marx Brother and get away with it. (114)
Allen W. Ellis writes in his article Yes, Sir: The Legacy of Zeppo Marx (The Journal of Popular Culture, Vol. 37, No. 1, 2003):
Indeed, Zeppo is a link between the audience and Groucho, Harpo and Chico. In a sense, he is us on the screen. He knows who those guys are and what they are capable of. As he ambles out of a scene, perhaps it is to watch them do their business, to come back in as necessary to move the film along, and again to join in the celebration of the finish. Further, Zeppo is crucial to the absurdity of the Paramount films. The humor is in his incongruity. Typically he dresses like a normal person, in stark contrast to Groucho's greasepaint and 'formal' attire, Harpo's rags, and Chico's immigrant hand-me-downs. By most accounts, he is the handsomest of the brothers, yet that handsomeness is distorted by his familial resemblance to the others — sure, he's handsome, but it is a decidedly peculiar, Marxian handsomeness. By making the group four, Zeppo adds symmetry, and in the surrealistic worlds of the Paramount films, this symmetry upsets rather than confirms balance: it is chaos born of symmetry. That he is a plank in a maelstrom, along with the very concept of 'this guy' who is there for no real reason, who joins in and is accepted by these other three wildmen while the narrative offers no explanation, are wonderful in their pure absurdity. 'To string things together in a seemingly purposeless way,' said Mark Twain, 'and to be seemingly unaware that they are absurd, is the mark of American humor.' The 'sense' injected into the nonsense only compounds the nonsense. (21-22).
In a eulogy for Zeppo written in 1979 for The Washington Post, columnist Tom Zito writes,
Thank goodness for Zeppo, who never really cracked a joke on screen. At least not directly. He just took it from Groucho, in more ways than one. ... If Groucho, Chico and Harpo were the funny guys, Zeppo was the Everyman, the loser who'd come running out of the grocery store only to find the meter maid sticking the parking ticket on his Hungadunga.
It turns out Zeppo did have one surprising fan, as revealed in Marc Eliot's 2005 biography of Cary Grant. Grant, a teenager performing in Vaudeville under his real name, Archie Leach, loved the Marx Brothers. And as Eliot puts it,
"While the rest of the country preferred Groucho, Zeppo, the good-looking straight man and romantic lead, was Archie's favorite, the one whose foil timing he believed was the real key to the act's success.