The 'Idol' host opened up to ET on Sunday about the unexpected royal appearance.
Following Sunday's new episode of Idol, Seacrest spoke with ET's Denny Directo, and explained how quickly the high-profile cameos came together.
"I have to tell you, I was driving in and my phone rings and it's the executive producer and she says, 'Well, we've got some news,'" Seacrest shared. "I thought maybe something happened [with a contestant]. She said, 'Well, the king and the queen are going to make an appearance!'"
"I said, 'This is so great. How?' 'Well I don't know. They're just gonna figure it out when Lionel and Katy are in some room in Windsor Castle,'" Seacrest continued, referring to Idol judges Lionel Richie and Katy Perry, who are in England after performing at the Coronation Concert.
"So apparently, Lionel and Katy are staying overnight? Like, I don't know what's going on. They've got they've got separate wing," Seacrest joked.
Charles and Camila's surprise appearance on the long-running singing competition series came at the start of Sunday's show and just hours after Perry and Richie's performances at the historic event.
The pair of over-the-pond judges, who were broadcasting live from the castle for their address, were met by the new monarchs who jokingly asked how long they were planning to use the room for.
Jokes aside, both Charles and Camilla thanked the singers for their "brilliant" performances and invited Perry and Richie to the concert's afterparty as soon as they wrap up their Idol duties.
ET also spoke with American Idol showrunner and executive producer Megan Michaels Wolflick on Sunday, and she opened up about how exciting and nerve-racking it was to make the special appearance come together during a live broadcast.
"We were sweating. It's been intense. Live shows are always intense, but then actually they were like, 'How about this? How about this? How about this?' And it made for a wild ride," she shared. "But it was really fun."
The coronation concert proved to be an exciting one for the judges, with Richie getting Charles up on his feet with his hits, "Easy (Like Sunday Morning)" and "All Night Long," and Perry moving Princess Charlotte to sing along to "Roar," before having the entire audience feeling like a "Firework."
Meanwhile, in the states, guest judges Alanis Morissette and Ed Sheeran filled in while Richie and Perry make their way back from the U.K. Morissette is also serving as a mentor for the Top 8.
American Idol airs live, coast-to-coast Sundays at 8 p.m. ET/5 p.m. PT on ABC.