A climactic plot structure will normally begin late in the story, very close to the climax of the play. It will normally cover a very short space of time, roughly a few hours or at most a few days. It occurs in a very restricted location such as one room and involves very few characters, up to a maximum of about 6 to 8. The plot is linear and moves forward without any form of interruption and there is no sub-plot.
By contrast, an episodic plot begins very early on in the story and moves through a series of episodes over a much longer period of time. It has several characters and several different locations. The plot of such plays is also marked by two or more different strands such as a sub-plot and a central plot.
Bearing these differences in mind, we can see that this play is clearly episodic in terms of its plot structure. The play has a long time span, moves between a number of different locations and begins towards the beginning of the action rather than just before the climax when Othello kills his wife. Such an episodic plot enables us to see Iago's scheme being hatched and how he works on so many different characters slowly but surely. Remember what Iago says about himself in Act I scene 1:
Heaven is my judge, not I for love and duty
But seeming so, for my peculiar end,
For when my outward action doth demonstate
The native act and figure of my heart
In complement extern, 'tis not long after
But I will wear my heart upon my sleeve
For daws to peck at: I am not what I am.
Having declared this about himself at the beginning of the play, an episodic plot structure allows Shakespeare to develop the character of Iago through seeing how he relates to various different individuals and how he hatches his plot to gain his revenge against Othello and various other characters.