Eminem's Oscars performance was among the biggest surprises of Monday's ceremony, met with confusion from both celebs and those watching along at home.

Now, it has been revealed the rendition of Lose Yourself took a carefully devised operation to keep it under wraps - with a clever ruse at the dress rehearsal likely explaining why the stars looked so baffled.

According to Robert Mills, a programming executive at ABC, which aired the Oscars in the US, the Academy had conceived a fake "back-up" rehearsal number to throw anyone who wandered in unannounced off the scent.

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Idina Menzel looked very confused by the performance - as did Billie Eilish. Photos / News.com.
Idina Menzel looked very confused by the performance - as did Billie Eilish. Photos / News.com.

Mills told Entertainment Tonight there were only a handful of people in on the surprise, which had apparently been in the works for years.

"Really, up until 48 hours before, I think maybe six people knew," Mills said.

"The Academy, us at the network, the producers and Eminem's team. And that was really it."

Eminem, who spoke to Variety following the performance, revealed that they only rehearsed once at the Dolby Theater, with Mills spilling that at the full dress rehearsals, Eminem's band even played a different rendition altogether.

This could explain why Idina Menzel and Billie Eilish in particular looked so confused - given they performed earlier in the night and were probably at the dress rehearsal in question.

Eminem received a standing ovation following the performance. Photo / AP.
Eminem received a standing ovation following the performance. Photo / AP.

Given Eminem's performance on the night came at the end of a montage celebrating iconic film songs, striking up from the very last beat of what was playing on screen (a snippet of Lose Yourself from 8 Mile) a different ending was rehearsed to trick people into thinking the live tribute would instead be a salute to Saturday Night Fever.

"When we did dress rehearsals, there was a whole alternate beat where it was just Eminem's band playing You Should Be Dancing by The Bee Gees with John Travolta in the background, so that's how much it was kept a secret and surprise. There was a whole alternate ending to that beat," Mills explained.

"If anybody happened to wander in and see it they would have been like, 'Oh yeah, there's been a switch with a salute to Saturday Night Fever," he confirmed. "It was a fake-out."

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Eminem performs Lose Yourself at the Oscars on Monday. Photo / AP.
Eminem performs Lose Yourself at the Oscars on Monday. Photo / AP.

Eminem – real name Marshall Mathers – has since commented that it was not his idea to keep the performance secret, but when the plan was floated, he agreed that it would be "kinda dope" to keep it unannounced.

According to Mills, that had always been the plan.

"I want to say the first time it was ever talked about was after Justin Timberlake opened with Can't Stop the Feeling in (2017). There are just these songs when you have an iconic performer opening with an anthem like that it just gets everybody pumped up," he said.

"We talked about it with the Academy that Eminem, the fact that he never performed that song and it's such an anthem, and it's one everybody knows and it won the Oscar. Wouldn't it be great to figure out a way to get him to come and do that?

"This year the producers … had a relationship with Interscope, his label and really made it happen."

Eminem in the Academy Award-winning film 8 Mile. Photo / AP.
Eminem in the Academy Award-winning film 8 Mile. Photo / AP.

After deciding not to attend the ceremony in 2003, leaving his collaborator Luis Resto to accept the award from Barbra Streisand, Mathers' performance on the Oscars stage was a long time coming.

While reports at the time said he refused to attend because he didn't want to perform a censored version of the hit track to the audience, yesterday he told Variety the rumours were "not true at all".

"I never even thought that I had a chance to win … And also, back at that time, the younger me didn't really feel like a show like that would understand me," he explained of skipping the ceremony.

"I remember being kinda confused about why I was even up for one."

Immediately after taking the stage, Mathers' tweeted "Sorry it took me 18 years to get here" to his 22.9 million followers.

Eminem performing at Auckland's Western Springs in 2014. Photo / Supplied.
Eminem performing at Auckland's Western Springs in 2014. Photo / Supplied.

Instead of attending the night almost two decades ago, Mathers said he went to bed early so he could get up in the morning and take his daughter Hailie to school.

That year, Eminem beat out Renée Zellweger and Catherine Zeta-Jones' I Move On, from Chicago; Caetano Veloso and Lila Downs' Burn It Blue, from Frida; U2's The Hands That Built America, from Gangs of New York; and Paul Simon's Father and Daughter, from The Wild Thornberrys Movie.