There are a plethora of elements that comprise comedy. They range from (inexplicably) destructiveness (hopefully with no permanent harm) to understatement to chases. Some of the elements of comedy in Sure Thing range from the premise to timing to detachment.
The premise itself is comedic: a fella tries to meet a gal, is shot down with "Sure thing" ... and tries again ... and again ... and again. Sure thing. Detachment itself is a comedic element present in several scenes, such as this early short one:
Bill: Is this seat taken?
Betty: No it's not.
Bill: Would you mind if I sit here?
Betty: Yes I would.
Bill: Oh. (Bell)
Another element is the comedic device of sounding of the bell at the end of each scene, rather like the gong of disapproval in a talent show: You've been dinged. A related element is incongruity in various forms (the bell would serve as one form of incongruity). An example of incongruity relates to the title of the book Betty is reading. Bill incongruously identifies The Sound and the Fury as having been written by Hemingway. Ding:
Bill: What's the book?
Betty: The Sound and the Fury.
Bill: Oh, Hemingway. (Bell)
Of course, you know that it was written by ... Faulkner.
Two other related elements are daring and culturally impolite responses. Bill shows real daring--in a socially timid society (what happened to calling cards and proper introductions: Go talk to her. I can't go talk to her. Go talk to her. I can't go talk to her.)--in trying to approach Betty. Some of Betty's responses are culturally impolite:
Bill: Do you come in here a lot?
Betty: Why are you asking?
The timing may be the finest element of comedy in this one-act play. There is a suspenseful tempo to the advance retreat rhythm. Bill's advance isn't straight forward. As soon it looks like he's on a roll, he gets dinged and has to start afresh:
Bill: Do you come here a lot?
Betty: Every once in a while. Do you?
Bill: Not much anymore. Not as much as I used to. Before my nervous breakdown. (Bell)
Betty: ... None of which interests me, mister!
All throughout, the comedy element of a mixture of serious and light is present in subtle degrees. For example, the discussion(s) of different time zones and missed connections addresses a seriously important issue in today's American culture in which estrangement is a chronic social ill, yet it is brought up in the midst of light romantic comedy.
Other comedic elements within Sure Thing are mild insults and putdowns; no permanent harm being done by the detachment, hostility and insults; one-liners; irony; and understatement ("Bill: Ah-ha. (Bell)").