Iago makes Othello an offer. He will get Cassio to say it himself. Stand out of sight, he says, and watch his face.
Othello draws back behind a pillar. At that distance the face and the gestures carry and the words do not. The man who chose the place was Iago.
Cassio comes in shortly. Iago says a name in a low voice and Cassio bursts out laughing. From where the other man stands, only the laughter shows.