"The impact, the response, has been incredibly overwhelming and very moving," Dempsey says. "It was nice to be able to have it work out this way."
"There's so many souls that we have lost right now," he adds of the nearly quarter of a million people who have died from COVID-19. "The thought of having angels around us is very comforting, certainly to me."
Dempsey's surprise appearance came after a get-together with Pompeo over the summer, where they brainstormed ways to encourage people to wear a mask.
"We hadn't spoken in quite some time, so we were playing catchup and she goes, 'What do you think about coming back on to the show? Here's our idea of what we want to do for season 17,'" Dempsey recalls. "I was like, 'What a great opportunity for people. How great it would be for the fans, really, to see everybody reunited in this sort of bizarre way.'"
Dempsey credits both the series' writers and its showrunner, Krista Vernoff, with coming up with the concept of Derek visiting Meredith in her "COVID dream."
Once the agreement was made, keeping the surprise a secret became the number one focus.
"Nobody knew about it other than... some people at ABC, Krista, and then Ellen and myself and that was it. And then we started talking to the directors, which would be Debbie Allen, who's amazing, and Kevin McKidd, who's one of the directors that I worked with," he says of Allen and McKidd, who both also star in the series. "That was it."
"No one knew until I showed up on set that day," Dempsey continues. "We were really concerned that somebody would take a picture and it would get out somehow, but we managed to keep it a secret, which was pretty phenomenal."
As for his continued appearance in future episodes, Dempsey teases that Derek shows up "throughout the season" when he "comes back to visit" Meredith.
When ET's Nischelle Turner spoke to McKidd, who plays Dr. Owen Hunt on Grey's, he recalled the "paranoia" the cast and crew felt while trying to keep Demspey's return a secret.
"We were on the beach shooting this, and at one point, we saw this boat out on the horizon and it stayed there for a bit too long. We were all like, 'That's a paparazzi boat! That's a paparazzi boat!' But luckily it wasn't a paparazzi boat, but we really thought it was," he said. "There was a lot of paranoia about keeping this thing secret. It was stressful. We had to lock down that whole beach, so nobody was allowed on it. It was a whole production, like a military operation."
For all the work that went into it, McKidd thought it was an "amazing" surprise.
"I'm glad it came off because I think it gave -- we've all been through so much with COVID, it was like a huge Christmas gift to the fans," he said. "And I think it was worth all the effort."