Earlier, Adele had performed the James Bond theme "Skyfall" for a small audience of the show workers.
It was an extra starry, musical day at the Dolby Theatre.
Adele took the stage first Friday, followed by the cast of "Les Miserables," singing together of the first time.
Oscar nominees Hugh Jackman and Anne Hathaway, along with co-stars Russell Crowe, Amanda Seyfried, Helena Bonham-Carter, Sasha Baron-Cohen, Eddie Redmayne, Aaron Tveit and Samantha Barks rehearsed their performances on the Oscar stage. They were backed by members of the musical's stage productions from London and Broadway.
"Les Miserables" director Tom Hooper sat in the front row of the theater as his cast sang.
Moments earlier, Adele dazzled the tiny audience of show workers with her performance of the James Bond theme "Skyfall."
"I need a lot more reverb on me," she said after her first run. "You might need to get a new reverb machine."
The 24-year-old multiple Grammy winner arrived wearing a black tunic, black leggings and flats, with no makeup and her hair in a ponytail.
"I'm going to have very high heels on the night, guys," she announced from the microphone, sipping tea between verses.
"Do you need the dresses?" she asked, and a team of stylists brought out the gowns Adele is considering for her Oscar performance.
The dress producers favored? "It's very heavy — I mean I struggle to stand in it," Adele said. "Come and feel how heavy it is, so you don't think I'm a wimp!"
She performed her Oscar-nominated song five times before leaving the theater. "It's been good, yeah?" she asked producer Neil Meron, who nodded in approval.
Just after Adele wrapped, the star-studded "Les Miserables" cast took the stage. Hathaway chatted with Bonham-Carter as Jackman sang a capella. Then Hathaway checked her microphone with a quick verse.
"Ooh, that was flat," she said.
The entire cast assembled for a final run-through when Jackman spontaneously began singing "My Bonny Lies over the Ocean."
"My bonny lies over my daddy," the ensemble responded, breaking into laughter.
Other stars rehearsing Friday included Jennifer Hudson, who is set to perform a song from "Dreamgirls" at Sunday's ceremony.