Oxford English Dictionary:
A. adj. Of a face, look, etc.: expressionless, impassive. Of a person: having such a face. Also transf., applied to speech, behaviour, etc.: detached, impersonal.
Buster Keaton, Jack Benny, Jerry Seinfeld, Ice Cube, Eddie Murphy, Will Smith
Mark Twain, from "How to Tell a Story" (concerning the American deadpan style)
. . . There are several kinds of stories, but only one difficult kind—the humorous. I will talk mainly about that one. The humorous story is American, the comic story is English, the witty story is French. The humorous story depends for its effect upon the manner of the telling; the comic story and the witty story upon the matter.
The humorous story may be spun out to great length, and may wander around as much as it pleases, and arrive nowhere in particular; but the comic and witty stories must be brief and end with a point. The humorous story bubbles gently along, the others burst.
The humorous story is strictly a work of art—high and delicate art—and only an artist can tell it; but no art is necessary in telling the comic and the witty story; anybody can do it. The art of telling a humorous story—understand, I mean by word of mouth, not print—was created in America, and has remained at home.
The humorous story is told gravely; the teller does his best to conceal the fact that he even dimly suspects that there is anything funny about it; but the teller of the comic story tells you beforehand that it is one of the funniest things he has ever heard, then tells it with eager delight, and is the first person to laugh when he gets through.
Chinese-Americans are sometimes stereotyped as aloof and humorless. Might this be another version of the deadpan? (Early Anglo-Americans would have characterized this look as "inscrutable.")