What are the four important soliloquies in Othello and what do they mean?
Shakespeare makes extensive use of soliloquies in his plays to share the inner thoughts of his characters and to reveal crucial information with his audience. Through them, characters inform the audience by effectively talking to themselves and this drives the plot and develops character. In Othello, the audience knows that there is more than meets the eye in the character of Iago, and it is ironic that it is the honesty he displays in such words as "I am not what I am" (I.i.66), wrapped up in contradiction, which has the characters believing in him, oblivious to his wicked manipulation.
Important soliloquies include those mentioned in the previous answers - Act I, scene iii and Act II, scene i- wherein Iago's self-interest and misinterpretation of circumstances feed his obsession with revenge. They reveal Iago's intention to take advantage of Othello's "free and open nature" (I. iii. 393) and to use Cassio's obvious friendship with Desdemona to mislead Othello because Iago says, "I know his trumpet" (II.i.176) meaning Iago knows that he can use Othello's weaknesses - and jealousy - against him.
In Iago's soliloquy in Act II, scene iii, Iago is relishing his satisfaction because it has been so easy to use Cassio and Roderigo to further his plan. He is pleased that Cassio is set up and that he only has to "fasten but one cup upon him (Cassio)" (44) in order to further his plan and Roderigo is a "sick fool" (47) who is so love-struck that he will do anything for Iago in the mistaken belief that he will win over Desdemona. This soliloquy gives the audience a better understanding of Iago's developing plan.
In Act III, scene iii, Iago is still working on Othello's weaknesses. As Iago leaves, Othello confirms Iago's "exceeding honesty" (262).This soliloquy reveals Othello's insecurities and he doubts himself "for I am black" (267). It also contributes to his confusion. His fears are allayed when he sees her and he regains his faith in her. Unfortunately thanks to Iago, it will not last.
BY Act V, scene ii Othello is so convinced of Desdemona's betrayal that, despite his love for her and his feelings of regret, he cannot allow her to "betray more men" (6) and so he intends to kill her whilst at the same time acknowledging his conflict because "so sweet was ne'er so fatal" (20). This soliloquy from Act five confirms the over-riding effects of Iago's manipulation and the very misleading "ocular proof" (III.iii.364).
In the soliloquy in Act I, scene iii, lines 320-341, Iago lays out his plan for revenging himself on the Moor. He has just convinced Roderigo to continue pursuing Desdemona, despite her marriage to the Moor, because Iago argues she'll soon grow tired of the Moor and want a younger man. He advices Roderigo to sell all his land so he has enough money to follow Desdemona and Othello to the wars where Roderigo can court Desdemona. Judging by the first few lines in the soliloquy in question, it would also appear that Iago plans to con Roderigo out of money in some way:
Thus do I ever make my fool my purse.
For I mine own gained knowledge should profane
If I would time expend with such a snipe
But for my sport and profit. (lines 320-323)
In the above, Iago is saying that he always manages to con money out of fools and that he would be wasting his talents in helping a fool like Roderigo if Iago didn't financially profit from it.
From there, Iago continues to profess how much he hates Othello and to speak of the rumor that Othello has seduced Iago's own wife. Due to this rumor, as well as other reasons, Iago feels justified in revenging himself on Othello.
Next, he begins to plot in what way he can make use of Cassio. Iago argues that since Cassio is good looking and charming and Othello is very trusting, it will be very easy for Iago to convince Othello that Cassio is seducing Desdemona.
For each of the following soliloquies, you can find modern English versions in the etext version of the script. For now, I'm just giving a capsule summary.
One comes at the end of Act I, scene 3. It is Iago alone on stage, and he spells out the meaning quite clearly: he hates Othello, and here is why.
One comes at the end of Act II, scene 1. Again, it is Iago alone on stage, and he spells out his plans for Cassio, and what he really thinks is the case (and will happen).
One comes in Act III, scene 3. Again, it is Iago alone on stage. This time he plans out what he'll do with the handkerchief (a key plot element).
Another comes in Act IV, scene 1. After Othello falls into his trance, Iago is alone on stage, and plans to talk to Bianca (another plot point).