The extras in Marx Brothers films

The Marx Brothers films Duck Soup (1933) and The Cocoanuts (1929) introduced me to the gravitational pull exerted by the extra. This may sound a little counterintuitive since our attention in these movies is clearly drawn to the antics of Groucho, Chico, and Harpo. Each brother leaves a particular sound-imprint on us: Groucho's waterfall/open sewer of puns, asides, and commentary; Chico's fabulously accented voice morphing the English language into new shapes and misunderstandings; Harpo's resolute silence, interrupted occasionally and decisively by his horn and harp. Who then are these countless people who appear in the background of every Marx brother film? What is their function?

Here are some lines from an essay I've published about the extra in films by the Marx Brothers:
On whether Zeppo, the bland fourth Marx, is in fact closer to being an extra than a brother:
Is Zeppo in fact part of the fraternity? Do we count him in? Though bearing the same name as his celebrated brethren, Zeppo seems adopted. Groucho’s cigar is more important to the Marx effect- has greater family resemblance, so to speak –than does Zeppo.

About Harpo's silence: 
Harpo is in some ways “the actor” as envisioned in the dream of the extra: silent, yet spectacularly on view; saying nothing yet having all the great lines. Harpo's silence seems to spectacularly ingest the silence of the extra, in the same way he eats buttons off the bellboy’s suit in The Cocoanuts. The bellboy, a subordinate both in the hotel and in the Hollywood system, stands staring into space as quiet as a statue, as Harpo quietly picks buttons off and munches on them one at a time. One senses the chafing of two silences here. The bellboy’s mouth, not his vest, is on the verge of becoming unbuttoned. How does he keep from screaming? Harpo’s silence is a feast, a willful opportunity to sample objects. Desire, hunger, lust are part of this now of Harpo’s silence, whereas the bellboy/extra is in a holding pattern, expectant, awaiting the sound of the bell after which he gets his name. So Harpo’s silence tests this other silence, pushing the extra to an outburst, an act of surprise, a line in the script.

On reducing your co-star to an extra:
Groucho’s approach to dialogue –if this is the word –resembles Harpo’s. Harpo’s gags, true to their name, seem aimed at choking speech out of his interrogator, leaving such figures as the lemonade vendor in a kind of breathless agitated state over the onslaught of Harpo’s physical humor.  In a similar manner, Groucho addresses his deafness to his interlocutor. His commentary on what they are telling him begins with this immunity to communication. Commentary, like Harpo’s gags, undoes the speech of the other. In short, Groucho seems determined to turn every actor on stage into an extra.

On Margaret Dumont:
Margaret Dumont offers something new in the history of cinema: the face of the acoustically bewildered witness. She neither laughs at nor dismisses what Groucho says. Instead her eyes circle and widen as if she were distracted by the insectlike nonsense of Groucho’s words (he’s called Rufus T. Firefly in Duck Soup for a good reason). Her face is a kind of eardrum. We search it out to discover if and how Groucho is understood and to detect the emotion of response in which the speech event continues. Dumont’s face is not the blank unagitated face of the extra: she in fact demonstrates endless surprise at him but without this ever becoming unseemly, without it ever sharpening into shock