We also move from a play in which true evidence is accepted only very hesitantly to a play about rashness and the eager acceptance of insufficient, entirely circumstantial, evidence.

In Othello as in Oedipus Rex we jump right into the plot, into the middle of Iago's machinations. In fact, we jump directly into the middle of a conversation:
I.i.1-26

Othello's problem in his play is pride and jealousy and the incapacity to withhold his horrid action. Othello is just as incredibly irrational as was Oedipus, jealous, a warrior, a man who ultimately trusts only those with whom he has seen battle.

The issue of "bonding" in battle, in fact, may go a long way toward explaining why Othello is so willingly led on by Iago, for they have fought together, as Iago says, when he's complaining about the election of Cassio:
I.i.27-33

Cassio, on the other hand, Iago contemptuously tells us, is little more than
a perfumed dandy.

Othello may have selected Cassio for political reasons; he is, after all, a
Florentine among Venetians--a representative of the most sophisticated
Italian state--while Iago and Othello are not just outsiders but mercenaries and thus not entirely to be trusted, no matter how much they are needed.

By the first 100 lines, then, Shakespeare has established the main plot, the sub-plot with Roderigo, and let us see precisely the sort of man Iago is:
I.i.55-65

Roderigo should see, but fails to see, the sort of man he is dealing with,
that Iago will be as dangerous to him as he is to Othello--and thus we see
the sort of person Roderigo is, and how Iago is using him as well as
Othello.

From this point forward, in other words, we have a good idea of how Iago is going to behave; now we are just watching to see how it all works out.

By the end of I.i, look at how much we have to go on: Iago is a villain, we
know, Roderigo probably a fool. Brabantio disapproves of Roderigo as a
suitor for his daughter, Bianca, but approves even less of her having
married Othello. And note how Iago announces the marriage in both racial and sexual terms:
I.i.86-92

A few lines later he gets even more vulgar in his description of Othello and Desdemona:
I.i.114-115

We also learn in this scene just how important Othello is to the Venetians:
I.i.145-55

Brabantio may be important (he tells us at line 179 that he can "command at most" houses), but Othello is, for the moment, even more important and is thus going to get away with marrying Desdemona.

In the second scene, we see directly just how two-faced Iago is, for
there he prompts Othello to be ready to fight Brabantio over Desdemona (and it is no coincidence that Iago swears by "Janus," the god with two faces, at line 33).

When Brabantio enters, we see him virtually overcome by hatred--and accusing Othello of using witchcraft to capture Desdemona, as if it is simply impossible that this young, wealthy, aristocratic--and white--woman could possibly love a man like Othello:
I.ii.62-79

As we see in the next scene, however, Othello's and Desdemona's views are
quite different from Brabantio's:
I.iii.128-70, 179-87

He's heroic, and Desdemona has fallen in love with him because of his valor. We might see that as an insufficient reason for love, but Shakespeare's audience would have seen it as quite sufficient.

We need to note particulary a couple of things at this point: first, be
aware that Othello has been given the opportunity to answer charges against him--which is considerably more than he allows Desdemona to do when he decides that she has been unfaithful.

Second, before he leaves in total disgust, Brabantio makes a statement that will later come to haunt Othello: the Desdemona should not be trusted:

Bra. Look to her, Moor, if thou hast eyes to see;
She has deciev'd her father, and may thee.
I.iii.289-90

By the end of Act I, then, the suspicions that Shakespeare established in
the very first scene are brought out directly: we see Iago gulling Roderigo,
and we learn of Iago's specific plans:
I.iii.306-380

Mostly, though, we learn here something of Iago's real motivation: he
believes that Othello has cuckolded him (ll. 362-64)--and we get a reiteration of that belief in the first scene of Act II:
II.i. 271-78

We are certainly safe to say that Iago is evil, but we need to note as well
that his actions, irrational though they may be, are not without motivation. He may be wrong about what he assumes, but his assumptions nonetheless drive his actions.

Act II opens with an announcement that the danger is over; the Turkish
fleet, like the Spanish Armada in the late 1580's, has been destroyed by a
storm; the political danger is now passed--and Othello faces a far more
difficult, pernicious, danger. That danger begins with his very safety, for
when we learn that he is safe from the storm, it is in conjunction with
Iago's safe landing in Cyprus. The latter brings with him his wife, Emilia, and
Desdemona.

Note in this scene how Shakespeare begins with the end of the war with the Turks and ends with the beginning of Iago's "war" against Cassio. And
thoughout, he has Iago remind us again and again of just what he intends to do:
II.i. 164-71, 192-194

Iago convinces Roderigo that Cassio is in love with Desdemona, thus making it easy to provoke him to attack Cassio--then gives us a bit more of his motivation and plotting at the end of the scene:
II.i 262-288

This is another crucial soliloquy, for here Iago reveals still more
motivations--lust, jealousy, fear of cuckoldry; his own evil leads him to
think ill of everyone, even his own wife.

Notes

Arithmetician: While Iago may indeed be deriding Cassio as one who has only
"book learning" and no real experience in battle, this particular epithet
has special meaning: if, indeed, Cassio is proficient at mathematics (more
geometry than arithmetic), he would be especially valuable to Othello, for
he would be adept at calculating trajectories for cannon--an artillary
officer, in effect. At a time when guns were coming more and more important
in battle, the common footsoldier became less important than those who could
handle artillary. Thus we see another possible reason for Iago's irrational
behavior: he has been made outmoded by the newer forms of warfare, and
Cassio's being chosen as Othello's lieutenant is proof of his descending
position in the military hierarchy.
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In Act III, scene iii (lines 207-9), convincing Othello that Desdemona has
been unfaithful to him, Iago recalls what Brabantio had said, using it as
part of his proof that she is not to be trusted: