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.