Julianna Margulies and George Clooney

The on-screen reunion that many felt needed to happen before ER could properly end its 15-year-run will come to fruition after all.

Though neither NBC nor Warner Bros. have yet to officially confirm the news, sources tell TVGuide.com that both George Clooney and Julianna Margulies — aka one of the medical drama's premiere on-screen couples, Dr. Doug Ross and Carol Hathaway — will resurface in one episode before County General shutters its doors on April 2. Both actors are reportedly filming their scenes this week, on a closed set.

Clooney last checked into ER in May 2000, making an unbilled, end-of-episode appearance so as to properly usher off Margulies' nurse Hathaway. The secretly filmed scene had Carol approaching a dock, where Doug waited for her, smiling yet silent.

Just last week at the TCA Winter Press Tour, TVGuide.com asked NBC entertainment president Angela Bromstad if the extension of ER's final run by three episodes might afford John Wells additional time to talk Clooney into an encore. Her response: "Ask John Wells." Days later, the ER boss actually dismissed the idea, saying that Clooney — whose film career has soared over the past decade — is "too busy."

Indeed, Clooney's publicist as recently as September pulled a Debbie Downer by deflating talk of the movie star's possible drop-by, saying, "It is something he has already done."

Weeks later, Margulies herself scoffed at the idea of returning during ER's final season, explaining, "I left Carol Hathaway in the best scenario possible. I can't imagine making it any better."

Already this season, Anthony Edwards reprised his role as Mark Greene (appearing in a "new" flashback with Laura Innes, aka Weaver); Alex Kingston put in an appearance as Mark's widow, Elizabeth, on Jan. 15; and Noah Wyle is due back as Carter for the final four episodes (though that could change in light of the extended order).

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