Cassio's trust in Iago is what makes Cassio so easily fooled by Iago. His character is confident and kind, and Iago uses him to the best of his ability, as seen throughout the play. His anger towards Cassio is what gives him the impetus to initiate the plan and ultimately remove Cassio from his role as lieutenant. Cassio has no doubts and automatically assumes responsibility without questioning anything. His trusting character is what Iago uses because of how oblivious and easy to fool he is. Together, all these explanations create a perfect combination that Iago can take advantage of and deceive Cassio throughout