"The most important characters in Macbeth are the witches." Evaluate this statement with direct reference to the play.

In order to argue that the witches are the most important
characters in Shakespeare's Macbeth, I believe you would have to
center on a certain issue/theme in the play.  You would have to go beyond the fact that
they are catalysts or that they put the idea into Macbeth's mind to become
king.

You would need to center on the issue of
predestination or fate.  You would have to center on the idea that the witches really do
know the future, and that they, or fate, or God control the
future. 

The issues of predestination and free will were
important in Shakespeare's day, due to the Protestant reformation, and the issue of fate
has been debated and studied since at least the Greeks.  This is what you would need to
center on in order to argue that the witches are the most important characters in the
play.

Think of it this way:  if the witches know the
future, then isn't the future set?  If the witches know the future, then what choice
does Macbeth have?  None, according to the argument you will need to
make. 

There's a fine line between knowing the future, and
determining it.