Let us introduce theater critic Addison DeWitt with some of his own humble opening lines:
DEWITT: My native habitat is the Theater. In it I toil not, neither do I spin. I am a critic and commentator. I am essential to the Theater.
Broadway critics can make or break a show with their reviews, and DeWitt knows it. Yeah, he's condescending and full of himself. His sarcastic commentary introduces the story to us and he assures us he knows "all about Eve." He's on to Eve before anyone else, because that's what he does for a living—he watches performances and he knows one when he sees one.
Addison knows the seedy underbelly of the theater as well as anyone:
ADDISON: It is senseless to insist that theatrical folk in New York, Hollywood and London are no different from the good people of Des Moines, Chillicothe and Liverpool. By and large, we are concentrated gatherings of neurotics, egomaniacs, emotional misfits, and precocious children.
He knows how to take advantage of all this. We see him at the party coaching Miss Caswell to throw herself at the somewhat decrepit but powerful producer Max Fabian:
ADDISON: You see that man, that's Max Fabian, the producer. Now go do yourself some good.
MISS CASWELL: Why do they always look like unhappy rabbits?
ADDISON: Because that's what they are; now go and make him happy.
Being a critic keeps Addison on the margins for the first half of the movie. An actor, director, and playwright aren't exactly going to hang out with a critic outside of the theater, are they? However, he's brought into their social circle unwittingly, like a demon accidentally summoned by a Oujia board, when Margo introduces Eve and Addison at a party. It's a match made in heaven.
Margo pushes them together because of Eve's "great interest in the theater," and because Margo, who calls Addison "Rasputin," wants to punish Eve. Spending time in the company of a blowhard like Addison DeWitt is punishment indeed. However, Eve sees this as an opportunity. She knows Addison is disliked and manipulative, so when Addison writes a column about Eve that insults Margo, Eve easily blames it on Addison.
Big mistake. Addison recognizes a fellow psychopath. He decides to take control of Eve. Addison's a creepy control freak, even when deciding dinner. Eve, in a bath towel, says to Addison, "You take charge." He responds, "I believe I will." We respond "EEW!"
Addison's creepiness goes full tilt when he tells Eve, "you will belong to me." He doesn't even like her, though. Why does he want her? In his words:
DEWITT: You're an improbable person, Eve, and so am I. We have that in common. Also a contempt for humanity, an inability to love and be loved, insatiable ambition— and talent. We deserve each other.
As creepy as he is, we kind of have to agree. However, he goes too far when he slaps Eve for laughing at him. That's when Eve realizes what she's in for.
ADDISON: Remember as long as you live, never to laugh at me. At anything or anyone else, but never at me.
What does Addison get out of this relationship? A sense of power and control over the most esteemed actress in the business? He couldn't do that with Margo—she's too tough and too authentic—but Eve's met her match in Addison.