Tools of Characterization
Character Analysis
Actions
Actions are interesting in Othello: characters that do the same thing can actually be actually quite different.
Think about how Othello and Iago both kill their wives. Same action, very different circumstances. Othello is misinformed and delusional. Iago, on the other hand, is cognizant and desperately trying to save face or inflict revenge.
Same deal with Desdemona and Emilia: Desdemona lies to Othello about missing the handkerchief, but only because she doesn't want to hurt his feelings and is probably certain she'll find it soon anyway; Emilia, on the other hand, lies to Desdemona about the same handkerchief because she's a thief who put her husband before her friend.
So it seems that, if we are to interpret actions as tools of characterization, Shakespeare is asking us to look a little beyond the action itself to the motivation behind it. This is where we get information about the characters. And that information (Iago is a villain, Othello is gullible, Desdemona is naïve, and Emilia just wants to be loved) is consistent with everything else we know.
Family Life
This tool of characterization gives us an opportunity to compare marriages. Othello has fallen deeply in love with Desdemona and married her because of that love. His marriage is new, and he is still getting used to giving up his independence to be with her.
Iago, on the other hand, has been married to Emilia for a long time, and it's hard to see any love in their relationship. He seems tired of her, and only talks to her to insult her or to get her to do things for him. Cassio is unmarried and comes off as something of a playboy. He's not that into his girlfriend, Bianca, a prostitute who's obsessed with him, and is happier smooching the married Emilia.
Habits
Othello is extremely self-disciplined. He's obviously a favorite with the Duke, and deservedly so, it would seem, as he doesn't even protest when a call to war interrupts his honeymoon. Iago gets the job done, but he's always scheming behind the scenes—he uses his job as a screen for his machinations.
Then we've got the storytelling habit: Othello tells true stories to Desdemona that make her fall in love with him, whereas Iago uses false stories to break up that very marriage. Again, these habits reveal the natures of the characters: Othello is genuine, Iago is not.
Occupation
Othello is the number one general of the entire Venetian fleet, and he rose to his position in Venice against all odds—he's the only black foreigner with a leading military position in Venice. Iago is one of Othello's back-up guys, and despite many years of experience as a soldier he is passed over for promotion. Cassio is Othello's new number-two guy, even though he's more of a brainy technical guy than a battle-ready warrior.
This is an important point as it gives us some potential, or at least partial, motivation for Iago's bad behavior: he's jealous of Othello's success at work. Following the same line of reasoning, we can see why Iago doesn't mind taking out Cassio along the way.
Sex and Love
Othello is passionately in love with Desdemona, but the way he views her sexuality shifts as Iago puts not-so-nice thoughts in his head. Othello is not inherently distrusting of women; instead, he is convinced to be so.
Let's face it; Iago's a pig—he claims all women are sex-obsessed and survive in the world by sleeping around. In his words, women "rise to play and go to bed to work" (2.1.114). Iago's ideas about female sexuality eventually rub off on Othello.
Speech and Dialogue
Othello, a Poetic Speaker
For a man who makes his living waging war, Othello's speech is consistently eloquent and poetic. Stopping a group of men from getting into a fight, Othello calls out, "Keep up your bright swords, for the dew will rust / them" (1.2.76-77).
Iago, a Man of All Tongues
Iago, in contrast, varies his speech according to the people who surround him. Often he is deceptively bland, his plain speech earning him the title of "honest Iago" even as he spins his web of lies and mwah-hah-hahs his way through life.
At times, he is filthy with his language, as when he tells Desdemona's father that she and Othello are "making the beast with two backs." Again, he chooses to speak this way because it does the trick and gets the job of the hour done. But Iago's language is at its most masterful as he convinces Othello that Desdemona is cheating on him. Iago's lines are full of sudden stops, apologies, broken-off lines, as he delicately feigns reluctance to tell.
Cassio, a Man of Unremarkable Tongue
Unlike these two silver-tongued men, Cassio's speech is unremarkable. His only skill is in crafting pretty compliments for the ladies.
Desdemona, a Lady With Purpose
Desdemona's speech is full of passion and conviction. From the opening scene where she explains her love for Othello to a hostile audience, to her defense of her own innocence when her husband gets abusive and accusatory, Desdemona always speaks boldly and from the heart. She never hesitates to share her mind, and what she says is straightforward and eloquent.
Emilia, a Bitter Wife
Emilia, on the other hand, says little, and her speech only grows inspired when she is talking bitterly about the cruelties of men to women. Emilia's instinct, when Desdemona is being falsely accused, is to curse and look for someone to blame.