Vin Diesel says Fast & Furious 7 was “the hardest and most important” film he’s worked on.

The action movie’s production was delayed after franchise star Paul Walker died suddenly in a car accident last year.

And Vin confesses working without his longtime colleague was a huge challenge.

"Fast 7 [was] the hardest movie I've ever had to make and in some ways the most important movie I've ever had to make,” Vin told to ET Online.

Paul’s siblings, Corey and Caleb, took on the late actor’s role, filling in scenes that still needed to be shot after he died.

Vin claims shooting the rest of the feature with them brought up all kinds of strange emotions.

“On some levels it was very cool to know his brothers were supporting this and wanting to help. On another level, they triggered memories, unintentionally at times,” he explained to Extra. “There’s no one like Paul Walker and there was only one Paul Walker, and it is a very tricky task for an actor to both mourn and be in the process of that stage of mourning and also have to pretend that person is still there. It made the experience that much more arduous.”

Vin found himself in tears when production recommenced on Fast & Furious 7 months after Paul’s passing.

“I remember the first day we came back to set. I am in a scene where I just run over Jason Statham’s car, and I must have gone through three boxes of tissues,” he recalled to Extra. “I am supposed to be in killer mode, but just being in the vehicle that was synonymous with Paul and myself, it was so hard to get through.”

Fast & Furious 7, which also stars Michelle Rodriguez, Dwayne Johnson and Tyrese Gibson, reaches theatres next year.