After co-star Paul Walker had died suddenly, the cast of the franchise was left in shock and grieving the death of their friend.
"It was, needless to say, the hardest movie I've ever done," Diesel told Access Hollywood. "I remember when the tragedy happened, my mother said to me, 'It's not fair that you have to mourn in front of the world.'"
While the cast had a hard time completing the film without Walker, Diesel said that having to do it also gave them a form of closure they may not have gotten otherwise.
"It's very, very weird," he said. "The good thing is, because we took all the time, the movie is a true testament to everyone coming together and wanting to honor both the saga and our brother, and there's something very beautiful about that."
Diesel will next be seen in Guardians of the Galaxy, a project he said helped him heal after Walker's death. He explained, "I'm really fortunate that I have both of these movies, and there's something very, very healing about playing a character like Groot that celebrates life after a tragedy like the one that happened."
You can see Diesel next when Guardians hits theaters on Aug. 1. FAF7 will be out April 3, 2015.
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