In order for Othello to believe that Cassio and Desdemona had an affair, Iago plays on the Cassio's trait of friendliness. As Cassio is from Florence, appose to Venice, he is more affectionate than the Venetians are used to, such as kissing and hugging the other characters. An example of Iago using Cassio's kindness as part of his plan is when he says 'I will gyve thee in thy own courtship' in Act 2, Scene 1. I think that Jamie Dornan could portray a friendly character because he looks like an approachable man.
Additionally, Cassio has to be attractive otherwise Othello wouldn't be able to understand why Desdemona slept with him. Therefore Cassio could be played by Jamie Dornan as he is attractive and was voted one of the most attractive men in the world following his most recent role as Christian Grey.
Finally, Cassio has to look very formal looking as he is from Florence. Evidence for this can be seen in Act1, Scene 1 when Iago is listing why Cassio shouldn't have been appointed as lieutenant saying 'a great arithmetician', rather than a soldier. This gives the impression that Cassio must be of a higher ranking than Iago and this must be clearly presented by the way the actor looks. Previously, I have casted David Tennant as Iago and I think that the two men contrast enough in how they look to represent the contrast in where they are in the social hierarchy. This also explains why I wanted to choose an actor who had light hair, in contrast to David Tennant's dark hair, to emphasise that David Tennant is the antagonist and that Jamie Dornan will play the role of the innocent character.