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).