Of all the characters introduced in the Grey's Anatomy pilot, model-turned-doctor Izzie Stevens (Katherine Heigl) looked like the one with the poorest dramatic construction (aside from Alex Karev, but textbook jerks can afford to be a little shallowly developed at first): naive, blonde and sickeningly optimistic, she wouldn't have lasted very long. Fortunately, the fourth episode digs a little deeper into her psychology, and permits Heigl to build the foundations of one of the show's most rounded, compelling performances (as the Emmy for her work in Season Three confirms).
Right at the start, she is shown in a more three-dimensional light than usual, as she playfully embarrasses George by using the bathroom while he is in the shower. This adds to his frustration and makes him exclaim: "I'm not your sister!", although the real irritation will come at work: Izzie's last modeling job is published, prompting Alex to make her the laughing stock of the entire hospital. In addition, a patient recognizes her from a magazine photo and consequently refuses to be treated by her. As for the other more interesting intern, Cristina experiences a bad day of her own when she is forced to take care of a former nurse instead of helping Dr. Shepherd remove a set of nails from a guy's head (yep, you read correctly: nails in the head).
That Cristina is the most fully rounded of the characters was obvious from the pilot episode (however, George and Derek are more fun to watch), while everyone else starts developing a realistic personality as the seasons pass. The briefest and most successful evolution is Izzie's, a transformation that reaches its peak when she tells off her judgmental patient and argues to perfection that a few shots of her in sexy lingerie do not necessarily reduce her medical skills. The anger and true humanity shown by Heigl in that scene are astounding, even touching, to the point that she gets away with another tough scene (the now infamous locker-room strip) without once looking ridiculous.
With such gifted supporting players, the series can justify the overall daftness of its protagonist. This concession doesn't apply to other shows, though: can you imagine The Sopranos with Tony being the least compelling character?