Although there is no stage direction calling for action, what does Lagos speech clearly indicate that Cassio is doing? How does lago plan to "ensnare" Cassio?

Iago convinces Roderigo that Desdemona will soon tire of the Moor. He says then the only man in Roderigo's way will be Casio. Iago is able to use his clever tongue to convince him of this,

"who stands so eminent in the degree of this fortune as Cassio does? A knave very voluble, no further conscionable than in putting on the mere form of civil and humane seeming, for the better compassing of his salt and most hidden loose affection. Why, none, why, none!"

Iago convinces Roderigo to start a quarrel with Casio. Iago will make sure that Casio is drunk and the ensuing comotion will humiliate and disgrace Casio in front of Othello.