Hamlet's decision to stage a play accelerates the action of the drama. Up to that point he had been procrastinating about fulfilling his duty to kill Claudius. He did not quite understand what was wrong with him. But when the players arrive and he decides to test Claudius by putting on the play he calls "The Mousetrap" he seems more inspirited, more motivated. And when he feels satisfied that his uncle is truly guilty of his father's murder and that he was not being deceived by the Devil disguised as his dead father, Hamlet seems to become a changed man. His new decisiveness and determination can be seen in the way he behaves toward Rosencrantz and Guildenstern in Act 3, Scene 2, and then the way he makes Polonius look like a fool in that same scene. They show that Hamlet has grown psychologically in a very short space of time. He will not let people manipulate him or play "mind games" with him. He is on his way to see his mother, and he intends to tell her how he feels about her marriage to Claudius and her shamefully inadequate mourning period for his father. We feel that Hamlet is a stronger, more mature man now. His development from an introverted, melancholy college student into a man worthy to be a king is the main motif of Shakespeare's play. At the end of Act 3, Scene 2, Hamlet speaks a revealing soliloquy.
'tis now the very witching time of night,
When churchyards yawn, and hell itself breathes out
Contagion to this world. Now could I drink hot blood,
And do such bitter business as the day
Would quake to look on. Soft! now to my mother!
O heart, lose not thy nature;
My tongue and soul in this be hypocrites
How in my words soever she be shent,
To give them seals never, my soul, consent!
His mother, of course, has known him since infancy, but she finds that he has changed considerably just since the aborted enactment of the play within a play. She says:
Why, how now, Hamlet!
Have you forgot me?
She senses that her ascendant position as his mother is in jeopardy. She would like to avoid the showdown and the tongue-lashing she suspects is coming. But Hamlet tells her:
Come, come, and sit you down. You shall not budge.
You go not till I set you up a glass
Where you may see the inmost part of you.
Hamlet is speaking about a metaphorical mirror, but his frightened mother, who thinks her son is mad, gets the idea that he literally intends to cut her open with his sword and make her look at her own intestinal organs in a mirror as she bleeds to death. Naturally she calls for help. And naturally the hidden Polonius echoes her cries. Hamlet is bewildered. He thinks he has walked into a trap, one probably arranged by Claudius. He has his mother calling for help in front of him and another voice calling for help behind him. If the guards should arrive promptly he would be carried off to a prison cell and might never come out of it alive. So he draws his sword and kills Polonius.
We know that this is all a misunderstanding because Hamlet assures himself in the soliloquy quoted above that he has no intention of hurting his mother.
let not ever
The soul of Nero enter this firm bosom.
Let me be cruel, not unnatural;
I will speak daggers to her, but use none.
So the outcome of the play Hamlet calls "The Mousetrap" strengthens his character and motivation, and seems to speed up the action as it leads to the deaths of Polonius, Rosencrantz, Guildenstern, Ophelia, Laertes, Gertrude, Claudius, and finally Hamlet himself. Act 3 is like the summit of a mountain from which everything careens unstoppably downhill to an inevitable conclusion when Hamlet finally accomplishes his mission by stabbing his wicked uncle with the poisoned foil.