Because Othello by William Shakespeare is a play we do not normally have direct access to the thoughts of the characters made apparent by a narrator. Instead, we overhear dialogue and infer from the dialogue the internal states of characters. The limitations on this inference are that the characters might not be truthful, or might be deluding themselves or lacking self knowledge. There is one literary device in drama that exposes internal thoughts. That is the device of the soliloquy, in which a character stands alone on stage and thinks aloud, as it were, to him or her self, being overheard, as it were, by the audience. An example is Iago's speech:
... let me see now:
To get his place and to plume up my will
In double knavery—How, how? Let's see:—
After some time, to abuse Othello's ear
That he is too familiar with his wife.
Thus we can assume we have access to the internal thoughts of all characters who have soliloquies. Both Othello and Iago have extensive soliloquies. Although Desdemona has several monologues, and appears to speak truthfully, she does not have soliloquies.