Vin Diesel: 'Fast 7' Was The 'Hardest' Movie I've Ever Made

An emotional Vin Diesel opens up to ET about the loss of Paul Walker and why working with Paul's brothers on Fast & Furious 7 didn't make finishing the film any easier.

"It was validating in a way that his family cared so much about the saga and the continuation and the completion of Paul's last work," Diesel says of Walker's brothers Caleb and Cody.

First Photo: Paul Walker’s Brothers On Set Of ‘Fast 7’

The two brothers joined the Fast family to help shoot the remaining scenes left in the latest film following Paul's untimely and tragic death last November.

While promoting his upcoming film Guardians of the Galaxy, Diesel also explains that having his brothers on set was as "cool" as it was difficult.

"It didn't make the process any easier," Diesel admits. "When you've worked with someone for 15 years and you know each other's idiosyncrasies and you've created magic together for so long, it's a strange thing to mourn that person and simultaneously pretend that person is next to you.”

VIDEO- Vin Diesel Comforts Paul Walker Fans At Crash Site

Of the emotional experience, Diesel adds, "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."

Fast & Furious 7 hits theaters April 3rd, 2015.