Well, in Act II scene 3 we see Macbeth in a very difficult position. He of course already knows that Duncan is dead, and yet, to avoid casting suspicion upon himself, he needs to present a credibly shocked response when he is urged by Madcuff to go in to the King's bedchamber and see for himself what has happened to Duncan. What is interesting to think about is how "convinced" we are about Macbeth's performance when he emerges from the bedchamber. Note what he says:
Had I but died an hour before this chance,
I had liv'd a blessed time; for, from this instant,
There's nothing serious in mortality;
All is but toys; renown, and grade, is dead;
The wine of life is drawn, and the mere lees
Is left this vault to brag of.
This speech has something of a rehearsed quality about it and arguably could be considered unrealistic. It is hard to imagine that Macbeth could come up with such eloquence so quickly if he really had been surprised by the death of Duncan. Note too the way that Macduff's response is contrasted with Macbeth's: Macduff is struck almost speechless with horror - he cannot bring himself to even describe what has happened and must send the others in to see for themselves. This juxtaposition adds an extra note of inauthenticity to Macbeth's too-polished words.
In Shakespeare's Macbeth, Macbeth and Lady Macbeth make a enormous show of grief over Duncan's murder. And more than this, Macbeth kills the King's servants before they can wake up and insist that they did not kill Duncan.
Macbeth and Lady Macbeth plot the murder of Duncan when he comes to visit them in their castle. They drug the King's servants, and then plant evidence on them to make it look as if they killed Duncan. When the King's body is discovered by Macduff, Lady Macbeth insists that it cannot be true and then swoons (faints). Macbeth goes with Lennox to investigate.
When he returns, Macbeth says he wishes he had died an hour before such a thing had been done, specifically Duncan's murder.
Then Macbeth announces his regret over the fury which drove him to impulsively kill the two alleged murderers. When Macduff asks why he did so, Macbeth (cunningly) responds that under such horrible circumstances, who could not have done so—anyone with such affection as he had for Duncan would have done the same. Macbeth says:
Who can be wise, amazed, temperate and furious,
Loyal and neutral, in a moment? No man....
...Who could refrain,
That had a heart to love, and in that heart
Courage to make 's love known? (89-90, 97-99)
Macbeth makes a great show of being emotionally distraught over Duncan's murder.
All the men present decide to sit down and discuss this terrible event to see if they can find the root of this traitorous deed.