Mr. Stevens is a robot overlord wearing a butler costume. Analysis done.
Oh, we jest. This is what Mr. Stevens would call "banter." Mr. Stevens doesn't understand what sarcasm is, or joking. What Mr. Stevens understands is being prim and proper, paying attention to detail, and being Mr. Dignified.
Stevens's New Year's resolution has been the same thing for as long as he was old enough to count down until midnight (figuratively speaking: Mr. Stevens probably goes to sleep at nine p.m. sharp): to be the World's Best Butler.
The butler is one of those super-quintessential artifacts of Englishness, about as English as afternoon tea or beefeaters. The idea of the butler packs the same kind of culturally symbolic punch as, say, a small-town Texas sheriff does for America or a beret-wearing chef does for France.
Here's a quote about quintessential Englishness: "(The English) are more focused. More precise. We're always one step ahead. We have a certain style. An eye for detail. And we're obsessed by power. Stiff upper lip is key."
Is that Austen? Dickens? Is that from a listicle called "16 Stereotypes About The English?" Nope. It's from a fairly recent Superbowl commercial for Jaguar. Yep—that is a quote from an advertisement for an English car. A bunch of English dudes signed off on those stereotypes, so we're going to feel free to use 'em.
Because dang if Stevens the butler doesn't check all of those dang boxes. Right down to the fact that he's obsessed with power:
[…] a reply to the effect that those of our profession, although we did not see a great deal of country in the sense of touring the countryside and visiting picturesque sites, did actually "see" more of England than most, placed as we were in houses where the greatest ladies and gentlemen of the land gathered. (1.5)
But Stevens doesn't just want to be in proximity to power; he wants to gain power. Being a butler is indicative of a rigid class structure. He not only serves his master/mistress, but he presides over all the "underservants." Butlers in general (and Stevens in particular) occupy an exact space in their social hierarchy.
He also keeps that key stiff upper lip:
"Miss Kenton, please don't think me unduly improper in not ascending to see my father in his deceased condition just at this moment. You see, I know my father would have wished me to carry on just now." (3.395)
Anyone who has read a Jane Austen novel or watched a gritty British miniseries knows that being stoic is to traditional Englishness as being white is to rice. Stevens takes it to a whole new level, though, by being calm in the face of his dad's death. Dude. Cry a little. Everyone would understand.
Stevens totally succeeds at being a great butler. What he's not very good at is being anything besides a butler. His job is so important that there is no room for a personal life: he is just as stiff in private as he is in public.
So when something genuinely tragic happens—such as the death of his father, or Miss Kenton's departure—he continues to keep calm and carry on, never sharing how he feels—even to himself. Not surprisingly, many of the tragedies end with a feeling of "triumph": as Stevens looks back over his career, he prides himself on coming through difficult personal and professional situations with that all-important dignity:
And let me now posit that "dignity" has to do crucially with a butler's ability not to abandon the professional being he inhabits. (2.55)
In other words, he never takes off his butler suit. We feel a little better using stereotypes ripped from a hilarious Jaguar commercial to describe Stevens… because he's trained himself to be so one-dimensional that it's almost as though he is the living embodiment of a stereotype.
But the key word there is "almost." There's still a bleeding, feeling man underneath that butler exoskeleton.
Moreover, as you might appreciate, their implications were such as to provoke a certain degree of sorrow within me. Indeed—why should I not admit it?—at that moment, my heart was breaking. (8.52)
After Mrs. Benn nee Kenton decides to stay with her hubby in her seaside town instead of moving back to Darlington Hall (who can blame her?), Mr. Stevens is totally crushed. It's after this point that he decides to try "bantering," his robot-speak term for conversing like a normal human being:
It is curious how people can build such warmth among themselves so swiftly. […] But, then, I rather fancy it has more to do with this skill of bantering. (8.85)
But Stevens isn't trying on his banter-hat for himself. Oh no. He's still too fused with his butler persona to do that. His new American employer, Mr. Farraday, enjoys light banter, so Stevens is going to put a spin on his consummate butler routine: he'll learn to banter to further please his American employer.
In fact, Ishiguro is probably making a point by having Mr. Farraday be American. Mr. Stevens's Englishness has served him quite long enough—it's time to try on a new culture's set of rules for a change. (And we're not just waving American flags and shouting USA! USA! over here—Ishiguro has a long history of writing novels that challenge traditionally English institutions, like the very creepy prep school in Never Let Me Go.)
Ultimately we don't know whether Stevens ever succeeds at mastering banter. We do know, however, that his future jokes couldn't possibly be worse than the ones he tries on for size in this book.