A student brings up Banquo and tries to relate the line of kings to the edge of doom to Dante's Inferno.  Which leads to a discussion of Banquo and the more general tension in Shakespeare between friendship and love, solved in the comedies but always part of the loss in the tragedies.  Considerations of this issue in Romeo and Juliet, A Midsummer Night's Dream, The Merchant of Venice, Hamlet, King Lear, and of course Macbeth and Antony and Cleopatra.