As the crew - including Carey Mulligan, Oscar Isaac and John Goodman - assembled on the steps of the theater before the film's gala screening Sunday night, Goodman realized he didn't have time to take a personal photo with his smartphone.
As officials urged the actors to take their seats, Timberlake grabbed Goodman's camera and darted through the crowd outside.
"As they were yanking us in I said, 'Give me your phone' and I jetted out there," Timberlake says. "I took a picture of it and brought it back in and gave it to him. Everything around here is late except for the screening of the movie.''
Timberlake also helped Isaac truly appreciate his Cannes moment.
"When we were all standing up on the steps, I was staring into the abyss of cameras and light," Isaac recalls. "And Justin was like, 'Look out to the right.' And there was this sea of people all looking to see this (movie) happen. I hadn't even seen that. I was like, 'Oh, man. It was huge.' "
Davis has won plaudits his role in the movie about the '60s folk scene, which is directed by Joel and Ethan Coen. Timberlake, too, has seen solid notices for his role as an earnest performer.
He says he would have "run craft services" to be part of a Coen brothers project and he is soaking up every moment in the south of France.
He had been to the festival to promote 2007'sShrek the Thirdat the behest of DreamWorks head Jeffrey Katzenberg, but "I think he just wanted to come to Cannes and come to the beach."
This is Timberlake's first time being part of a competition film, and he said he had his moment when the applause began at the end of the movie. Most of it was directed at Isaac who is in nearly every scene.
"The credits came up and there was this huge ovation for Oscar," says Timberlake. "He was in the eye of the storm and had the movie on his shoulders. It was a 'Voila!' moment. I was just really proud and happy."