At the beginning of Act 1, Scene 7, Macbeth has already spoken with Lady Macbeth about maintaining an innocent attitude to arouse no suspicion. This indicates they have begun plotting something. Macbeth contemplates committing the murder in Scene 7, but tells Lady Macbeth to "proceed no further in this business." (I.vii.31) Lady Macbeth chastises him and convinces him that they will not fail.
In Act 2, Scene 1, after Banquo and Fleance go back to sleep, Macbeth agonizes over carrying out the murder. Lady Macbeth sounds a bell and this prompts Macbeth to go through with it. As Lady Macbeth waits for Macbeth to return, she is anxious and gets startled by a screeching owl. She notes that when she put the daggers in Duncan's chamber she would have killed him, but he looked too much like her father.
Macbeth returns with the daggers and bloody hands. He agonizes because he heard "Macbeth does murder sleep'--the innocent sleep," (II.ii.34). Lady Macbeth tells him to put all guilty thoughts out of his mind. She tells Macbeth to wash the blood from his hands, but he refuses to return the daggers to Duncan's chamber. Lady Macbeth returns the daggers so that it looks like the grooms (Duncan's servants) had committed the murder:
Give me the daggers. The sleeping and the dead
Are but as pictures. 'Tis the eye of childhood
That fears a painted devil. If he do bleed
I'll gild the faces of the grooms withal,
For it must seem their guilt. (II.ii.51-55)
They both hear sounds of knocking and return to their sleeping chamber.