In addition to the other answers, consider the tension created in the first two scenes. In Scene 1, the witches gather to discuss what a rotten piece of work Macbeth is:
First Witch: ‘When shall we meet again,
In Thunder, lightning, or in rain?
Where the place?’
Second Witch: ‘Upon the heath.’
Third Witch: ‘There to meet with Macbeth.’
All: ‘Fair is foul and foul is fair
Hover through the fog and filthy air.’
The audience, therefore, already has an opinion of Mabeth before the man is even seen on stage.
In Scene 2, however, we hear the soliders and nobles discussing what a swell guy their leader is.
King Duncan: O valiant cousin! Worthy gentleman!
Sergeant: I must report they (Macbeth and Banquo, another soldier) were as cannons overcharged with double cracks, so they doubly redoubled strokes upon the foe.
Dramatic irony is created in the tension between the two portraits. Is Macbeth a hero or a villian?
When Macbeth entersthe darkened hall, and meets up with Banquo, who was unable to identify him in the darkened hallway. Macbeth refers to Banquo as “a friend.” This is ironic because Macbeth will prove to be nothing resembling a friend.