In Macbeth, why does Macbeth ask Banquo so many apparently casual questions about where he is riding?
The questions Macbeth asks Banquo in Act III, scene i appear to be casual, as if Macbeth is only concerned that Banquo not miss that night's banquet. In fact, they are very pointed questions. He wants to know when Banquo is leaving the castle and how far he will be riding. This will give him an idea of when Banquo will be returning. Macbeth needs this information so that he can pass it along to the men he has employed to attack Banquo and his son Fleance on the road and murder them both.
After his conversation with Banquo, Macbeth meets with the murderers. He tells them the following:
Within this hour at most
I will advise you where to plant yourselves,
Acquaint you with the perfect spy o' th' time,
The moment on 't; for 't must be done tonight,
And something from the palace;
This suggests that Macbeth has a general idea as to where Banquo will be, but he wants to plan precisely where the attack should take place to ensure success and to make sure Banquo and his son are not murdered near Macbeth's castle. After he figures it out, he will get back to them in no more than an hour.
Macbeth is planning to kill Banquo and his son Fleance. He needs to know Banquo's plans so he can set the murderers on him. Macbeth has been told by the witches that Banquo will father a line of kings (the Stuart line). Macbeth's ambition drives him to attempt to kill his friend and his friend's son so that the witches prophecy will not come true and his own line (if he should have one) will thrive. He fails, because Fleance escapes and joins the princes in driving Macbeth to his death at MacDuff's hands.
In the witches phophisize that Banquo will have a line of kings although he would not be king himself. Macbeth is told by the witches that he will become king. Logically, Macbeth should be concerned with Banquo's even move because he is worried for he will lose his throne-to-be.