In order for an actor to play a scene he has to know in which direction the scene is going and he knows in which direction it's going because he decides.
DB - The Vagabond
In almost every important scene an actor has he has at least one secret. A secret is something that gets revealed. It may not get revealed until the end of the play, unless it is revealed to the audience earlier.
In the thousands of years of dramatic literature some of the greatest scenes ever written are the scenes between Iago and Othello when Iago, starting from nothing but a simple question: "Did Michael Cassio, when you woo'd my lady, know of your love?" gradually plants a seed of suspicion in Othello's mind. Thereafter, taking advantage of every opportunity and chance encounter to insinuate his lies into Othello's perturbed mind while pretending to be his friend, he finally drives Othello into a state of blind rage and murderous jealousy "Arise, black vengeance, from the hollow hell!"
The beginning is simple, the end is horrifying, but all the subtle steps along the way, the dimensions, colors, pauses, latitudes, perspectives, leverages and twists are up to the actor and if the actor makes the right decisions about how the scenes progress the result will be tragic in the fullest sense possible.
There's a show business story about a production of Othello that was playing out west many years ago in which the actor playing Iago was so good some pioneer type in the audience took out his pistol and shot him. The actor wasn't killed, fortunately, but it sure stopped the performance.
I never played either Othello or Iago, but I did play Cassio, the innocent soldier that Iago blames it on and so I got to witness that scene every night played by Clayton Corbin and Charles Kimbrough. Since both actors had made strong decisions they had very powerful directions in which to go.
So much of acting is following directions. The playwright gives you directions, the director and the production give directions. But in the end it's the actor's own directions that make the role. No one else can act the part.
Another interesting aspect to Iago's role is just how and how many of his secrets he reveals as the play goes along. We have a macabre fascination watching that friendly guy destroy Othello and a few other people along the way. Our reaction to him is going to depend partly on how much of himself he reveals to us. If his decisions are right we can't help ending up hating him. But leave your pistol at home.