The stage comedian is probably the actor par excellence. He or she works alone, without a mask, at the mercy of the public and can only rely on their own resources. “I have never been as scared, as humiliated in all my life”, confessed De Niro after the long take that accompanied his monologue in King of Comedy.
De Niro was also the first to be charmed by Rupert Pupkin. Scorsese, who had given him the script at the time of Alice Doesn’t Live Here Anymore, did not feel ready. Only after Raging Bull was he convinced that the material was interesting enough, more personal than he had previously thought.
In reality, the director had been fascinated by Sid Caesar, Ernie Kovacs, (the Jewish comedians of the Borscht Belt) for some time. He has a passion for Bob Hope and Bing Crosby’s particulary nasty comedy and in particular for Always Leave Them Laughing (Roy Bel Ruth, 1949), Milton Berle’s no frills autobiography, surprising at times for its truth and bitterness can be found here. Mel Brooks or Lenny Bruce’s liberating laughter is absent. A second rate jester, Rupert is no misunderstood genius. He is only a king in the dialogues that he imagines having with his idol, Jerry Langford. His long monologue, which he is always postponing, is not proof of an exceptional talent. Rupert fascinates us only because he would be ready to sell his soul to the devil, for one night of being a star.
Not without malice, Scorsese swaps the parts assigned to his stars. For Jerry Lewis there is the humorless businessman, sullen to the extent of being misanthropic, whose eyes light up only when he flies into a rage. De Niro, on the other hand, until then dedicated to impressive dramatic roles, borrows some characteristics and even some gags from the “bellboy” who he has watched again and again in all his films to get into the part.
[…] King of Comedy worked in revealing things to each of the participants, starting with Scorsese who, having matured, recognised himself in the two lonely characters, both in the insider and the outsider.

Michael Henry Wilson, Martin Scorsese. Conversazoni con Michael Henry Wilson, Rizzoli, Milano 2006