William Bliss’ 1949 book The Real Shakespeare concludes with a “Shakespeare examination paper for reasonably advanced students.” It includes this question:

5. What character (and in what play) has the shortest part; appears only once (and that in a stage direction); and says nothing — and yet is essential to the plot?

I thought this might make a good puzzle — but I don’t know the answer! Bliss withholds his, and despite a lot of fumbling research I can’t find a significant player in Shakespeare who says nothing at all. Maybe the bear in The Winter’s Tale? I’ll leave it here as an open-ended riddle.

UPDATE: In the messages I’ve received, the most popular candidate is Banquo’s ghost in Macbeth. Banquo appears alive and speaks early in the play, but Macbeth has him murdered in Act III and his ghost haunts Macbeth silently thereafter, plaguing his conscience. In the stage directions, the ghost is called “Ghost of Banquo,” so arguably this is a distinct character.

09/13/2013 Another possibility: the corpse of Henry VI in Richard III. He appears once, carried in a coffin while Richard woos the mourning Lady Anne; he certainly says nothing; and his death and the victory of the Yorkists set off the events of the play. (Thanks, Josh.)

08/18/2015 An even better guess: Elizabeth I appears once, says nothing, and is onstage for less than 100 lines in Act V, Scene V of Henry VIII:

All the expected good we’re like to hear
For this play at this time, is only in
The merciful construction of good women;
For such a one we show’d ’em …

(Thanks, Joe.)