Matthew McConaughey, Reese Witherspoon share vocal secrets behind ‘Sing’

Matthew McConaughey, Reese Witherspoon and Scarlett Johansson really did burst into song for Sing (in theaters Wednesday).

Director Garth Jennings says it had to be the A-list actors’ own vocals for the movie about animated animals who compete in a singing competition: “There’s much more rooting interest knowing the actors are really doing this.”

Here are the stories behind their performances:

MATTHEW McCONAUGHEY (as Buster Moon)

Voicing the gung-ho koala who hosts a singing competition to save his struggling theater, McConaughey tears into Carly Rae Jepsen’s Call Me Maybe. Not pretty, but fun.

“I said, ‘Look, nothing’s written for me to sing, but if you come up with something, I’d love to give it a shot,’ ” McConaughey says. In the recording booth, Jennings asked about Call Me Maybe. “I’d heard the song, obviously,” McConaughey says. “So I said, ‘Leave the recording (going) and I’ll do it over and over 20 times and we’ll pick one out of there.’ ”

REESE WITHERSPOON (as Rosita)

As the overstressed housewife with 25 piglets, Witherspoon belts out three pop showstoppers: Katy Perry’s Firework, Bananarama’s Venus and Taylor Swift’s Shake It Off. But the actress wasn’t entirely confident in her singing, despite winning an Oscar as June Carter Cash in Walk the Line.

“I ran into Katy when I was recording Firework and Taylor when I was recording Shake It Off, and explained to both of them that they should brace themselves for my version,” Witherspoon says. “You sound really good in your car or your shower, but then you have to actually go into the recording booth and it’s hard to sing those songs.”

Her trick to nailing the vocals? M&M’s. “It just gives me a sugar rush.”

SCARLETT JOHANSSON (as Ash)

Johansson is a recording pro, having released two alternative rock albums. But she went hard core as the punk porcupine, especially on the original anthem Set It All Free. “That may be a musical departure for me, personally, but it’s the perfect representation of Ash’s experience.” Her studio secret was “lots of Throat Coat (tea) and gusto and letting go of my inhibitions.”

Johansson also gave the full version of Call Me Maybe: “I mean, it’s certainly catchy.”

TORI KELLY (as Meena)

The American Idol star surprised herself as the shy elephant with killer pipes who does a stunning version of Leonard Cohen’s Hallelujah and a breathy Happy Birthday. But Kelly reached personal vocal heights for Stevie Wonder’s classic Don’t You Worry ‘Bout A Thing. “I didn’t think I could hit those notes. I was stretching my vocal ability.”

Chalk it up to studio coaching, technique and lots of car practicing (“I feel hidden from people there”). It paid off: “When I found out Stevie Wonder heard it and loved it, I freaked out.”

TARON EGERTON (as Johnny)

Egerton, the Welsh star of Kingsman: The Secret Service, showed surprising vocal ability, earning the part of soulful gorilla Johnny after an Otis Redding-filled audition for Jennings. Singing Stay With Me “really stretched me,” Egerton says. “Sam Smith’s falsetto is really insane, but I gave it a go.”

Johnny brings the house down with I’m Still Standing by Egerton’s hero Elton John. Like Johnny, Egerton doesn’t play the piano (“I wish”), but he can nail Can’t Help Falling In Love on his ukulele.