Elements in Macbeth that were more or less likely to come from elsewhere.  Who played whom.  Robert Armin (and Will Kemp).  Johnson on whether the reference to Antony and Cleopatra (Macbeth's genius overmatched by Banquo's as Antony's is by Caesar) is an interpolation.  De Quincey on the knocking at the gate, and the effect that the juxtaposition of scenes has.