Final answer:
Iago uses Cassio's appointment as Othello's lieutenant and his alleged interest in Desdemona to spark Roderigo's interest in his plan to discredit Cassio.
Step-by-step explanation:
Iago uses several pieces of information to spark Roderigo's interest in his plan to discredit Cassio. One of the main pieces of information is that Cassio has recently been appointed as Othello's lieutenant. Iago convinces Roderigo that Cassio is not fit for the position and that he has been chosen over Roderigo unjustly. Iago also tells Roderigo that Cassio is interested in Desdemona, Othello's wife, and that he plans to use Cassio's infatuation to his advantage to ruin his reputation.