Although the 30th anniversary of the film “Clueless” (1995) has revived interest in Shawn’s performance as an avuncular teacher, the part for which Times Square tourists may most likely know him is Vizzini, the Sicilian would-be hit man of “The Princess Bride” (1987). His most famous line, “Inconceivable!,” turns up several times in the movie, and Shawn’s wildly varied delivery — distinctively nasal and lisping one moment, cocky and debonair another — made it a kind of meme long before the internet would spread it even further. It’s an earworm of an exclamation — nerdy, beloved, handy. People still shout it at him on the street as he walks by. Even at a Harvard reunion (he attended the school as an undergraduate), teenage children of his classmates called it out as they passed him, something to be reckoned with for someone who once told an interviewer that his goal as a human being, since childhood, was “to be taken seriously.”