How does the way that Act 1 is organised help us to believe in the change that comes over Macbeth?
1. We are introduced to the supernatural first (the witches).
2. Then, we hear of Macbeth's valor on the battlefield. He guts a traitor, which is ironic foreshadowing; he will resolve to be a traitor by the end of the act.
3. We see Banquo and Macbeth with the witches. They are obviously foils of each other. We know both can't be tragic heroes. So we conclude that Banquo is a good guy (will not be influenced by the witches); Macbeth will.
4. We see Lady Macbeth also casting off her her morality and inviting evil. Macbeth might not have gone to the dark side if it were just the witches giving him the idea, but now his wife will really trigger his ambition.
5. Macbeth admits that he's wanted the crown all along. So, we have an idea to be king (the witches), personal ambition to be king (Macbeth), and a plan to be king by killing the king (Lady Macbeth). This is a recipe for change: the brave Macbeth will become a traitor. (One aspect of Macbeth that never changes, however, is his violent nature. He's always been a killer).