When the audience knows something that the speaker in the play doesn't.
E.g. When Arthur Birling talks about Titanic being unsinkable and there being no possibility of war (page 7). This makes the Birlings seem foolish. JB Priestley wanted to make the upper classes seem naive (and for humour).
JB Priestley uses lighting to create specific moods for specific reasons.
E.g. before the inspector arrives there is soft lighting to reflect the happy mood. when the inspector arrives the lighting is harsh to relfect the grim reality he is about to impose on the Birlings (Act 1)
JB Priestley uses sound effects to add impact for the audience.
E.g. the doorbell signifies the arrival of te inspector. It also signifies the arrival of a reality check for the Birlings (page 10). It is described as a 'sharp ring'.
Detailed Stage Directions
As viewers we will be aware of the detailed stage directions only by what we see on stage. JB Priestley is meticulous in creating an impression of the Birlings through the stage directions given at the beginning of the play. This allows the audience to know what sort of people they are.
The entire play is set in the Birling's house. This is important as it conforms to the three unities of time, place and action. The single setting puts the spotlight on the Birlings and allows the audience along with the inspector, tp interrogate the family so there's no escape for them.
JB Priestley uses subtle hints throughout the play to suggest things about the characters.
E.g. Sheila wonders where Gerald was last summer (page 3), Eric is used to pouring himself a drink and is 'squiffy' and Mrs Birling states that young wives have to get used to errant husbands.
The Inspector, Place and Tension
The inspector himself is a dramatic device. he controls the pace and creates the tension between the characters and for the audience. A brooding, unflappable presence, he is totally in comand from start to finish. His language is characterised by bold assertions and penentrating questions.
Timing, Exits and Entrances
Timing of entrances and exits is crucial.
E.g. The inspector arrives immediately after Mr Birling has told Gerald about his impending knighthood and about how "a man has to look after himself and his own" (act 1)