Do the witches cause things to occur or do they make suggestions about actions which the characters freely choose to follow?

In Macbeth, the witches simply
provide information that leads Macbeth to murder.

The
witches chant incantations, and they can see into the future. However, when they give
Macbeth information, it is not the complete
truth, but half truths. In fact, it is not until the end of the play that Macbeth
realizes that he has believed their assurances that he will not only be king but be
invincible, and that they have really only told him half truths; in the killing of
Duncan, the King, Macbeth realizes he has lost his soul (because it is a great sin to
kill a king), and that he will not even have the chance to enjoy the position within the
kingdom that has become his with the murders he has
committed.

The Elizabethans believed that witches could not
make people commit evil, but that they could trick human beings
into committing crimes, thereby costing them their souls. (This is Hamlet's worry about
believing the ghost in Hamlet: is the ghost real or are the powers
of darkness trying to win his soul to its doom?)

The
witches do NOT cause things to happen in Macbeth. They give Macbeth
information that is only half true, and he chooses to believe their
predictions and commits the evil deeds. (Things like the earthquake, howling winds,
chimneys falling down, the King's horses attacking each other, etc., are not caused by
the witches. All of these things are reflections of the disruption of
order in the universe because a king has been killed and the wrong
person—Macbeth—is on the throne. These strange occurrences will not stop until the
rightful heir sits on the throne.)