Miller uses dramatic irony to create anxiety and tension in a critical scene in The Crucible.
Dramatic irony occurs when the audience, along with certain characters on stage, is aware of a particular piece of information which is hidden from another character (or characters) on stage.
When Mary Warren brings the doll to Elizabeth after returning from participating in the witch-trials, a moment of dramatic irony occurs. In this scene the authorities come to the Hale house and accuse Elizabeth of using voodoo to harm Abigail Williams.
Elizabeth is asked if she has any dolls in the house. When she denies the presence of any dolls saying that she does not keep any dolls in the house, the audience is fully aware of the fact that Mary Warren has just brought in a doll.
The authorities are at first unaware of the doll, which is a moment of dramatic irony as the audience is aware of something which a character on stage is not, though quickly the doll is discovered.
The use of irony here creates tension because the audience is able to anticipate the discovery of the doll before it takes place.
Another example of irony can be seen in John Proctor's character. He is introduced as an adulterous husband attempting to extricate himself from the last ties of his affair. Yet, he is also the person who stands up to corruption. He is a liar, but he ultimately is willing to sacrifice his life for honor and for the truth.