An Aside in any dramatic performance is much like a parentheses. For, additional information is provided to the audience. This information is given in a statement by an actor who often steps forward on the stage and directs himself/herself to the audience, revealing his or her private thoughts, reactions, or motivations. Presumably, the other actors on the stage are unable to hear what is being said. Audiences will notice that sometimes all action stops while an aside is spoken, or the action goes on behind the character delivering the aside who is uninvolved. Sometimes, too, two characters will speak to each other in an aside; this situation is much like one character whispering to the other.
In Act I, Scene 3 of Macbeth, the asides of Macbeth do much to reveal his inner thoughts, thus providing character development. When Ross comes to him and upon orders from the king bids Macbeth "Thane of Cawdor" (1.3.113) Macbeth incredulously wonders in an aside, "What can the devil speak true?" Up until this point, he may not have taken the three witches seriously, but now he wonders because their prediction that he would become Thane of Cawdor has come true. Then, in an aside to Banquo, Macbeth encourages Banquo to believe what the witches have told him because they have been proven true by him,
Do you not hope our children shall be kings,
When those that gave the Thane of Cawdor to me
Promised no less to them? (1.3.126-129)
But, Banquo in an aside to Macbeth cautions Macbeth against believing the witches, suggesting that their attempts may be devious:
...to win us to our harm,
The instruments of darkness tell us truths,
Win us with hones trifles, to betray 's
In deepest consequence. (1.3.132-135)
This aside is of great importance to the characterization of Macbeth because he begins to believe that reality and fantasy are equal. As equals, then chance may bring him good fortune without his "stir." But, Banquo does not trust in chance, so Macbeth tells him to think more about it, and they can "speak/Our hearts each to other" (speak freely about it) later.