Vin Diesel was so devastated after the death of his friend Paul Walker, he wasn’t sure if he could ever return to the set of “Furious 7.” But he found inspiration in another project that immediately followed in those early difficult days: the voice of the heroic sapling Groot in 2014’s “Guardians of the Galaxy.”

Diesel recorded Groot in his four-month hiatus from “Furious 7,” as Universal Pictures halted production to figure out how to finish the movie without Walker in November 2013. Diesel, who had trouble leaving his house because he was so stricken with grief, met with director James Gunn to talk about Groot’s role in the Marvel tentpole, which went on to gross $774 million worldwide for Disney. “That was healing for me,” Diesel says in a lengthy interview for this week’s Variety cover story. “To play a character that could reincarnate, rejuvenate and represent life as trees do.”

James Gunn, who directed “Guardians,” says that Diesel was in a lot of pain when he approached the character. “Vin was in a very sensitive place,” Gunn says. “It was not long after Paul passed away. I think the simplicity of Groot, the innocence of Groot and the creativity of Groot was a big relief for Vin.”

Diesel created Groot’s distinct sound on his own, and practiced it with Gunn over the phone. “It took a lot of time and meditation,” Diesel says. “He needed to have this guttural voice, without always being so menacing. I just played with it and played with it, and I was inspired by my 3-year-old son at the time, who I was playing the role for. I always imagined the character would have to connect to a child.”

Gunn says that Diesel pushed himself as an actor. “For every ‘I Am Groot’ in the movie, he said it at least 500 times,” Gunn says. “Not to mention he did it in all the different languages.” Diesel recorded Groot to a version of the film where the tree-like protagonist had been temporarily voiced by other actors (including Gunn himself). “But when Vin came along, the character suddenly became real and most people’s favorite character,” Gunn says. “Vin has so much to do with that.” Despite what appears to be Groot’s demise at the end of the first film, Gunn wants Diesel back for the sequel. “Who knows what form Groot will be in by that time,” Gunn says. “I certainly hope Vin is involved.”