This oral history includes many performers who became closely identified with "ER," but starts with Mr. Wells, the show's driving force from its outset.

In the Beginning

JOHN WELLS George [Clooney] was the first to be cast. I knew him from seeing him around the lot. Les [Moonves, now the CBS chief executive, then the head of Warner Brothers studio] had made a cast contingent deal for a crime show with George, but George showed up in my office and said he'd heard about our show, and he liked the part better than the legal show. He had a scene memorized, and he did it, and it was terrific. I said we'd love to have him. But George had to convince Les because the other was a lead role and this was a supporting role. George just told him: I want to do this one. Then as now George was very aggressive and very smart about managing his career.

http://www.nytimes.com/2009/03/22/arts/television/22cart.html?_r=1