"It's not like Grey's Anatomy, where you can do some surreal thing and viewers just accept it," said Edwards, whose character succumbed to a brain tumour after Season 8.
"I was dead."
The resurrection of physician Mark Greene takes place in flashback scenes that shed light on why ER chief Cate Banfield (Angela Bassett) returned to County General.
After 15 years, ER is closing shop, so story lines are wrapping up and original cast members such as Edwards, Noah Wyle and George Clooney will help send off the show in style.
Returning to the set brought back vivid memories of those first days on ER, Edwards said.
He came in with a box office resume (Revenge of the Nerds, Top Gun) but little TV experience.
Clooney liked to joke that he'd been in so many failed TV projects that this was his second stab on a show titled ER and set in Chicago.
But they all sensed that what they were doing could be ground-breaking.
"NBC felt we were doing something different, too, because they often said we were way too different for their comfort," executive producer John Wells said.
"But breakout hits only happen when you surprise the audience, and I think we did that."
Edwards said the series constantly challenged the actors, and he's proud of his work there, including an experiment in which the show aired live twice - once for the East Coast of the United States and once for the West Coast.
"I can point to a lot of those big episodes, like the live one, with pride," Edwards said.
"But what I'm most proud of is the consistent quality of the show."
When ER premiered in 1994, the series faced a challenge from another new hospital series set in the same city - Chicago Hope, which boasted a well-known cast and a well-known producer in David E. Kelley.
But ER drew more viewers, quickly becoming part of NBC's Thursday night juggernaut with established hit Seinfeld and another freshman series, Friends.
In its first seven seasons, ER was television's most watched drama.
"From the start, the series reflected the reality of the emergency room with a visual, writing and acting style that pays respect to that," Edwards said.
"You felt like you were watching a real world, with people coming and going, and the audience rewarded that with longevity."
Edwards now lives in New York, producing TV pilots and doing charity work, most notably building a $US15 million ($NZ22.9 million) hospital in Kenya through his Shoe4Africa foundation.
His only condition for returning to ER was that his $US125,000 salary be donated to the foundation.
Steven Spielberg, an ER executive producer, quickly matched the donation.
Wells also donated $US50,000.
Philanthropy has been a part of ER almost from the beginning.
Wells said the success of ER had allowed the show to point a spotlight on critical health issues both locally and globally.
"We turned some attention on the Congo and on Darfur, when nobody else was.
"We had a bigger audience than a nightly newscast will ever see, making 25 [million] to 30 million people aware of what was going on in Africa," Wells said.
"The show is not about telling people to eat their vegetables, but if we can do that in an entertaining context, then there's nothing better."
Not only did coming back to the show help his charity; Edwards said it also gave him great pleasure.
In fact, he said, slipping back into his former role was as easy as putting on an old sweater.
"It was strange and fun being in that trauma room.
"I spent so much of my life there," Edwards said.
"All these young actors and you're the old guy.
"I felt like the grandfather who gets to come play with the kid, spoil it and then give it back."
Linda Cardellini, who has played nurse Samantha Taggart for five years, said the current cast relished working with returning actors.
"It seems like senior year, and this was homecoming," she said of Edwards' appearance. - Susan C. Young.
