In Shakespeare's play Othello, what are some illusions Othello has about some characters?I have his illusion about Cassio being a drunk...

The most obvious illusion is that Iago is 'honest' and is
trusted totally by Othello.

The audience is clearly
convinced of Iago’s duplicity as this is laid out clearly from Act I scene
i:

Though I do
hate him as I do hell-pains,
Yet for necessity of present life,
I
must show out a flag and sign of love,
Which is indeed but sign. (Act I scene
i)

 However it is Othello’s
inability to see Iago’s true self, which is Othello’s fatal
flaw.

In Act I scene ii, Iago swears ‘by Janus’ – the
two-faced Roman god. Honesty is mentioned over 40 times in the play, and most often
connected with Iago. It is not just Othello who is deceived by Iago: every other
character in the play is duped by him too.