Which events in Iago's deception of Othello were planned and which were opportunistic?

The first incident, featured in Act I, Scene 1, in which Iago and his sidekick Roderigo inform Brabantio that Othello stole his daughter Desdemona was definitely planned. When the upset Brabantio, who is awoken from sleep in the middle of the night, discovers his daughter is indeed missing, he immediately takes steps to have Othello arrested, as indicated in his question to Roderigo:

Do you know
Where we may apprehend her and the Moor? 

Secondly, Iago disappeared during Brabantio's discourse with Roderigo in the above incident in order to be with Othello. This, too, was planned, as he told Roderigo:

Yet, for necessity of present life,
I must show out a flag and sign of love,
Which is indeed but sign. 

Iago wanted to convince the general that he is there for him and purposely bad-mouths Brabantio in the process. Othello believed his so-called "faithful" ensign. 

In Act II, Scene 3, in Cyprus, Iago and Roderigo set up Cassio and he is dismissed from his post as lieutenant by Othello. In this scene, Iago deliberately creates the impression that he is most loyal and trustworthy by speaking well of Cassio, when he had, in fact, set him up. Othello falls for his trickery and says,

Thy honesty and love doth mince this matter,
Making it light to Cassio. 

Iago then opportunistically uses Cassio's dismissal to advise him to seek redress through Desdemona by asking her to speak to Othello on his behalf. He is not satisfied with Cassio's dismissal. He wants to destroy Cassio completely and, in the process, destroy Desdemona and Othello. Once again, he sets up Cassio to create the impression in Othello's mind that he is being cuckolded by his erstwhile lieutenant and Desdemona.

In the fourth instance, Iago comments, 'Ha! I like not that,' when he and Othello chance to see Cassio slipping away from Desdemona's company on their arrival. Othello is immediately intrigued by Iago's comment and asks about it. Iago is very evasive but finally persuades Othello to pay close attention to interactions between Cassio and Desdemona. He also makes the general aware of a so-called "custom" Venetian women have of keeping a secret lover. He also makes Othello note that Desdemona betrayed her father and might do the same to him.

Othello is deeply troubled by Iago's assertions and treats Desdemona with disdain, brushing aside her napkin when she tries to wipe his brow. Emilia, Iago's wife and Desdemona's maidservant, picks it up and gives it to Iago. Othello later confronts Iago, threatening him if he should be deliberately tainting his wife's honor. Othello tells him he wants "ocular proof" of Desdemona's infidelity.

Once again, Iago smoothly slips out of a difficult situation by deliberately lying about an occasion when he had, supposedly, shared a bed with Cassio and he had, in a dream, cried out Desdemona's name, kissed him passionately and then extended a leg over his thigh. Othello is in shock and threatens to tear Desdemona into pieces. The pernicious Iago, seeing his advantage, refers to the handkerchief Othello gave Desdemona as a first gift, saying that he saw Cassio wiping his beard with it.

This is opportunistic, for Emilia gave Iago the napkin and he had planted it in Cassio's room where he would obviously find it, with no idea of how it got there or to whom it belonged. At this point, Othello is convinced of Cassio and Desdemona's guilt and vows to take vengeance. He kneels to make a pledge and Iago kneels with him, promising absolute allegiance to the general.

In one final, maliciously deliberate act, Iago sets Othello up to see the ocular proof he required. He arranges a meeting with Cassio and asks Othello to stay hidden and eavesdrop on their conversation. Iago then entices Cassio to speak about Bianca. Othello believes that they are talking about Desdemona and grows angrier the more he hears Cassio supposedly bragging about his affair with his wife.

It is also opportunistic for Iago that Bianca turns up, angrily waving Desdemona's napkin at Cassio and asking why he could insult her by asking her to 'take out the work' of his mistress's napkin. Othello is overwhelmed. He decides to kill Desdemona by smothering her in her bed, at Iago's suggestion, and makes Iago promise to kill Cassio within three days.