Playwrights love stories in which there are a number of contrasting characters because it makes it easy for the audience to get to know them and to understand their various parts in the continuity. Othello is an excellent example of what Lajos Egri in his authoritative book The Art of Dramatic Writing calls "orchestration," using the analogy of musical composition, and especially of opera. The two leading characters, Othello and Desdemona, could hardly be more strongly contrasting. One is a man, the other a woman. One is black, the other is white. One is a fierce warrior, the other sweet and gentle. Giuseppi Verdi saw the operatic potential in Shakespeare's play and adapted it into a famous opera. Verdi could see Othello as a baritone and Desdemona as a coloratura soprano and imagine their love duet when Othello begins with:
Put out the light, and then put out the light.
If I quench thee, thou flaming minister,
I can again thy former light restore,
Should i repent me; but once put out thy light,
Thou cunning'st pattern of excelling nature,
I know not where is that Promethean heat
That can thy light relume. When I have plucked the rose
I cannot give it vital growth again;
It needs must wither.
Shakespeare has even provided Desdemona with a little song:
"The poor soul sat sighing by a sycamore tree,
Sing all a green willow;
Her hand on her bosom, her head on her knee,
Sing willow, willow, willow."
There are good parts for the other actors as well. There are three female characters, Desdemona, Emilia, and Bianca, all very different kinds of women. Iago is one of Shakespeare's best creations, and he presents a strong contrast to the Moor. There are also good roles for Cassio and Roderigo. Shakespeare had a whole company of actors and must have always been on the lookout for stories that could utilize all of them, including the young men who played the female roles.
In addition to the well orchestrated cast, Shakespeare must have liked the settings. Very likely his company already owned many of the costumes they would need for the production. Costuming was of great importance in creating the illusion of time and place, since there was little else in the way of furniture, backdrops, props, etc., available to create the sense of time and place. Shakespeare's audience probably enjoyed the illusion of being transported to faraway places, as they were in Romeo and Juliet, As You Like It, Measure for Measure, The Tempest, Julius Caesar, and many others. Drama has always been escapism--offering escape from one's own problems through identifying with the problems of others. Othello is both a tragedy and a love story, and Shakespeare's audiences must have favored tragedies and love stories. No doubt Shakespeare was attracted to the story of a man who kills the woman he loves. It makes for wonderful drama and poetic dialogue.
And Shakespeare must have enjoyed writing the dialogue for Iago, whom Othello calls "Precious villain!" Iago is probably Shakespeare's best villain and one of his best creations. He is wicked, cunning, cynical, resourceful, daring, witty, opportunistic, fearless, adaptable, and articulate. A really precious villain.