What are the moments of expository dialogue in the film Prestige, starring Christian Bale?
If I understand your question correctly, you are asking "What are ..." because of the unusual cinematic technique used whereby three events are simultaneously occurring and are linked together by the voice-over of Cutter (Michael Cain), who is speaking from the third location, which is the witness stand of the Criminal Court in London.
[Excerpts from Prestige film script]
TOP HATS. Clustered in a small glade.
BORDEN (V.O.): Are you watching closely?
A SECOND BLACK CAT races into frame, HISSING, SPITTING, CHASING the first cat into the woods ....
Interior CLUTTERED WORKSHOP--DAY
CUTTER (V.O.): Every magic trick consists of three parts, or acts ...
[Man] stops at a [canary] cage. Weathered HANDS envelope the canary. Hands and voice belong to a man in his 60s--CUTTER.
CUTTER (V.O.): The first part is called the Pledge...
Backstage SCALA THEATRE, LONDON--NIGHT
STAGEHAND: Where'd you think you're going?!
BEARDED MAN: I'm part of the act, you fool!
CUTTER: Who was that?
The PROSECUTOR turns to face the witness in the box: CUTTER.
As you may be able to see from the excerpts of the expository scenes, the "moments of expository dialogue" are, in regard to Cutter's voice-overs, Cutter's testimony from the witness stand in London's Criminal Court. His voice-overs inform the scenes being shown of the workshop in day, with Cutter there with canaries, and of the Scala Theatre in night, with the Bearded Man going backstage and below the stage. The other moments of expository dialogue are from the scenes being worked out at the other times in the other settings. Each of these threads works itself out in the course of the movie and together reveal the present time events that will follow from the Court testimony given by Cutter.
Additionally, the opening shots of hats and cats set up what we are to learn later in the film about the reality of the trick called The Real Transported Man. Of course, the meaning of the opening shots of hats and cats won't be revealed until the conclusion of the film, but they do establish the psychological associations that thread throughout the film. In sum, the moments of dialogue in the exposition of the film are snatches, in medias res, of events at three locations, from three time periods, involving three sets of people, with Cutter the common connection between them and with the woods providing the psychological orientation of the dialogue and events.