Consider the imagery and the host/guest motif in Act I scene 6 of Macbeth.

The importance of this short scene lies in the way that
the imagery and the host/guest motif allows Lady Macbeth to show that she is following
her own advice that she has just given to her husband in the previous scene. Let us
remember her words to him in Act I scene 4, where she counsels him
to:

...look
like th'innocent flower,

But be the serpent
under't.

This of course
relates to the key theme of appearance vs. reality, that is so ubiquitous in so many of
Shakespeare's plays. Act I scene 5 shows that this is something Lady Macbeth herself is
an expert in. Consider the way that even the presentation of the castle lures in Duncan
whilst concealing something much more sinister:

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This castle hath a pleasant seat; the
air

Nimbly and sweetly recommends
itself

Unto our gentle
senses.

Both Banquo and
Duncan comment upon the "pleasant seat" of the castle, which is of course a great
example of irony, because something extremely unpleasant awaits Duncan
there.

Likewise, Lady Macbeth, as hostess, presents a
perfect image of a loyal and faithful hostess to her guest, protesting that Duncan does
her and her husband a great honour by staying with them in their
house:

All
our service,

In every point twice done, and then done
double,

Were poor and single business, to
contend

Against those honours deep and broad,
wherewith

Your Majestly loads our
house...

Everything conspires
to make Duncan believe that there is nothing wrong and nothing being plotted, whereas
all the time, we know as an audience that Lady Macbeth and her husband are plotting to
kill Duncan in their own home. The dramatic irony heightens our appreciation of this
scene.