I was reading the original script of the Marx Brothers' Duck Soup, written by Bert Kalmar, Harry Ruby, and Paramount contractee Grover Jones (who isn't credited on the finished film). The film was called Cracked Ice at this time. This page details some of the changes that the script underwent between the submission of this script and the start of filming a few months later. In essence, the story is the same, but lacking most of the routines that made Duck Soup what it was. The script was pumped up in two ways: first, by incorporating a bunch of routines written for a Groucho-Chico radio show by the team of Nat Perrin and Arthur Sheekman (who received an "additional dialogue" credit) and second, through routines and gags added by the director, Leo McCarey. For example, McCarey added the mirror sequence and several other bits that had been staples of the silent, gag-filled comedies he had done for Hal Roach.
McCarey is one of those directors whose work really deserves a more in-depth study than it has received. There's a very strong argument to be made that McCarey in the '30s was the best director in Hollywood, possibly in the world. The variety and quality of his work in that decade is exceptional: in one year, 1937, he made both a stunning screwball comedy, The Awful Truth, and the most heartbreaking, depressing drama about old age ever filmed, Make Way For Tomorrow (a movie that makes Tokyo Story look like a Shirley Temple picture). He made Duck Soup; he made Harold Lloyd's finest sound movie, The Milky Way; he made Love Affair (which he later remade almost scene-for-scene as An Affair To Remember) and Ruggles of Red Gap and the best of the Eddie Cantor musicals (The Kid From Spain) and the multi-star comedy Six of a Kind. He was versatile, he was adept at comedy and drama, and he could bring out the best qualities in any actor -- Cary Grant adapted some of his Cary Grant persona from McCarey's own personality, and comedians like the Marxes and Lloyd gave of their best when McCarey was behind the camera.
What makes it harder to evaluate McCarey's work is that most cinema criticism focuses on a director's visual style -- how does he compose shots, how does he pace the film, how does he move the camera -- or on his thematic obsessions. McCarey's style isn't displayed in his unfussy compositions or the subject-matter he chooses, but in the way actors act in his films. McCarey was one of the few directors who was able to take the freewheeling, seat-of-your-pants style of silent comedy and bring it to the new era of big-budget sound movies. He preferred to work without a finished script (it's doubtful that Duck Soup really had a completed script by the time he started filming) and work out the dialogue and staging on the set with the actors. The unique quality of his work comes from this, from the fact that the actors seem so loose and unforced, as if they're making the scene up as they go along -- because, in some cases, they were. And the essence of McCarey's style is that he would let actors do things in the finished film that would be outtakes from any other movie: cracking up with laughter, trying to get out of each other's way, trailing off in mid-sentence: his best movies seem "real," and the actors don't seem to be acting, whereas most Hollywood movies, then and now, are very regimented and artificial.
The other thing that makes it difficult to evaluate McCarey is that his best work was basically behind him after 1939; his '40s work was very successful but not up to his best (the two priest movies with Bing Crosby), and his '50s work included one of the most embarrassing movies ever made by a major filmmaker, the McCarthyite My Son John. So it's hard to point to McCarey as an example of sustained greatness, though his later movies do have their moments, and he retained his ability to work with actors (compare Cary Grant in An Affair to Remember with the phoned-in performances he was giving other directors that same year). But in his prime, he was something close to a mad genius.