What did it take to get Anthony Edwards to return to "ER," even though his character has been dead for six years?

Money. Lots of money. All of it going to charity.

In tonight's episode of "ER" (9 p.m., Ch. 5), which is in its final season, Edwards returns as Dr. Mark Greene. It is, of course, a flashback. And it involves the current head of the emergency department, Dr. Cate Banfield (Angela Bassett).

According to Edwards, several months ago he got an e-mail from executive producer John Wells indicating that the writers had "come up with a way of getting Greene back and we'd love to bring everybody back into the show for the audience's sake. And it makes such perfect sense how they did do it, but I certainly could not have pictured it before."

It was also "perfect timing" because Edwards was about to launch a fundraising effort to build "the largest children's hospital in Africa" through the charity Shoe4Africa.

"So when John Wells asked me, I said, 'I'm happy to, but don't pay me. Let's get Warner Brothers to donate to the hospital,"' he told critics on a conference call.

So, instead of taking a salary, the studio donated $125,000 to Shoe4Africa. Steven Spielberg, who's one of "ER's" executive producers, matched that amount, and Wells added another $50,000 of his own.

"So for me it was really great," Edwards said, because it allowed him to not only return to the show but to help the charity.

"It's exciting to me. And, not that I want to be Mother Teresa but, basically, an actor's life is that of a narcissist. I mean, you are completely focused on yourself as an actor. And to have that balance, it just feels really good."

The flashback episode is set shortly before Edwards left the show and, thus, shortly before Mark Greene died. It fills in the blanks on the Cate Banfield character, who's been somewhat of a mystery.

And Edwards said the experience "was really like going back to high school and in the best way. It was like going back to your favorite class, having your favorite teacher and there was no test that day."

He said that after "about half a hour ... it really felt like I'd never left. The same conversations were happening that were happening six years ago, the same dynamics between the crew members the same jokes."

And getting back into character was pretty much like putting on a familiar pair of shoes.