They discuss the feast ahead, resolve to praise Banquo at it, and then Macbeth reveals his fear of banquo and fleance. He reveals that he intends to commit another dreadful crime, he will not tell lady macbeth what it is, but asks her to praise it when it is achieved.
The initial hope that they would enjoy their reign together is now exposed as hollow.
He no longer shares his thoughts with his wife so she feels isolated. The affectionate term, 'dearest chuck' that macbeth uses for her seems to imply a shift in power.For lady macbeth this scene marks the start of her breakdown, she will take control one more time, at the banquet, after which she will be overwhelmed by remorse for the tide of evil she has helped unleash. Their roles are reversing.