"False face must hide what the false heart doth know".
Acting is a key theme of this play, from Lady Macbeth's early injunctions for her husband to seem something different to what he is, and cover up his murderous intentions with a false face, right the way through to Macbeth's own final nihilistic realisation that 'Life's a... poor player... on the stage'.
Shakespeare seems keen to relieve the tension and pressure of the drama of the pre-murder scene. Macbeth's approach early on, with Macduff at the door, is to answer in very short, almost monosyllabic phrases. The audience know, and he knows, what he's done, and there's no need for a big performance.
Good morrow, both.
Is the King stirring, worthy Thane?
He did command me to call timely on him;
I have almost slipp'd the hour.(45)
I'll bring you to him.
I know this is a joyful trouble to you;
But yet ’tis one.
The labor we delight in physics pain.
This is the door.
Later on though, his mask begins to slip. He kills the servants at the door for no obvious reason (other than, of course, hiding his own guilt) and then makes a very uncomfortable, awkward speech attempting to prove his innocence:
Here lay Duncan,
His silver skin laced with his golden blood,
And his gash'd stabs look'd like a breach in nature
For ruin's wasteful entrance: there, the murderers,
Steep'd in the colors of their trade, their daggers
Unmannerly breech'd with gore. Who could refrain,
That had a heart to love, and in that heart
Courage to make's love known?
Read it out loud and pause on every punctuation mark. Feel how nervous and awkward he is? Exactly. And Lady Macbeth has to cover up for him with a distraction: she faints. Except, of course, it is in fact a feint faint - a fake, a show, an act. The acting honours, for my money, go to her.