The answer to this question can be found in Act II scene 1, which is when Iago observes very closely the kind of body language that Cassio displays when he greets and talks with Desdemona. In an aside, he reveals to us why it is precisely that he is paying such close attention to such activity, and what he plans to do on the basis of it:
He takes her by the palm: ay, well said,
whisper: with as little a web as this will I
ensnare as great a fly as Cassio. Ay,...
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