ER turns 20 today and usually we are still talking about ER because it is the First Great Medical Drama, the Last Great Network Drama (30+ million viewers weekly), or because reruns, you know? But its most sustained success was the cast, exceptional from the beginning and over 15 seasons, including more than 25 principals.

The show was slow/fast, pretty and then bloody, an arterial spray of "realism" in the mass-market American TV machine. The formula was not endlessly repeatable, but even for ER's quicksandening storytelling decisions, it had no less conviction in the pressure of its own dramatics, which made diamonds out of almost everyone.

Conventional wisdom favors the first generation, Clooney and Wyle and La Salle and Marguiles, et al. And rightly: Given the show's wild success through the '90s, the tropes and images of their interpersonal chemistry are hard to escape. (Before The Good Wife's thing with elevators, there was ER's thing with elevators.)

But the truest test for a long-running ensemble drama is to look past those first few years, when the iron-clad contracts begin to weaken and people fall away. How a new group should not just fill in like carbon copies of the old, how they should be from the same family tree but not so badly bred that they fall into an uncanny valley, like our favorites but not.

NYPD Blue did the same thing, at the same time, at almost the same scale, but not as well. Maura Tierney's Abby Lockhart was no Carol Hathaway, though they both started out as nurses, and both had that deep, still-water vibe. (Tierney's strength under the weight of declining story material is an inversion of the grade-A material Marguiles got, but lacking no less in glory.)

ER's first generation was replaced by Tierney, Phifer, and Cardellini. That bred inspiration, or however else you make out the decision to bring on Shane West for a few years, almost immediately after he appeared in A Walk to Remember. Alex Kingston, in the middle years, occasionally hitting a major chord when you didn't even know she had her instrument. Maria Bello! For just one season, which in hindsight seems insane and just right. Angela Bassett, at the very end, long before it was cool. And Yvette Freeman, always.

(Guest stars are an entirely other post, but of course there is Bradley Whitford, Djimon Hounsou, Dakota Fanning, Cynthia Nixon [a cottage industry in these kinds of things], Stanley Tucci and on and on.)

It's an endless discussion, which is part of its joy. Some of my favorite castmember moments: When George Clooney, vying for America's heart, saved a boy from a storm drain; or that episode after Kellie Martin gets stabbed to death, a cornucopia of grief. (Years later, at the time, did people talk about George O'Malley getting "Kellie Martin'd" or "Omar Epps'd"? I can't remember.) Most important: The only thing I want out of life is for someone to look at me the way that John Carter looked at Peter Benton.

[Video via YouTube, images via NBC]

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