How we cite our quotes: (Chapter.Paragraph)
Quote #4
By the very nature of witticism, one is given very little time to assess its various possible repercussions before one is called to give voice to it, and one gravely risks uttering all manner of unsuitable things if one has not first acquired the necessary skill and experience. There is no reason to suppose that this is not an area in which I will become proficient given time and practice. (5.12)
Wow, does this sound a bit like Mr. Spock to you? Stevens really has no social skills whatsoever. For him, a joke ("banter") is a potential minefield of awkwardness. Unlike his other butler duties, bantering cannot be mastered through "time and practice."
Quote #5
These were, let me say, overwhelmingly professional in tone—though naturally we might discuss some informal topics from time to time. (6.14)
Stevens really misses talking shop with his butler buddies, who may—along with possibly Miss Kenton and Lord Darlington—be the closest thing he has to friends.
Quote #6
"Do you realize, Mr. Stevens, how much it would have meant to me if you had thought to share your feelings last year? […] I suffered all the more because I believed I was all alone." (6.50)
Like bantering, expressing feelings is definitely out of Stevens's comfort zone. Interestingly, Miss Kenton seems to suggest here that expressing feelings is not irrelevant to establishing a strong professional relationship.