The latest installment, "The Fate of the Furious," debuts in theaters Friday without its star.
Walker died in a car crash in 2013 just as he was completing filming of "Furious 7."
The crash, which was not connected to filming of the movie, also claimed the life of his racing team partner, Roger Rodas.
Vin Diesel told "Entertainment Tonight"
on the red carpet for the film's premiere that it's been difficult since losing his best friend, whom he nicknamed "Pablo."
"A lot of people go, 'What's it like making a movie without Pablo' and that's not the right question," Diesel said. The real question, he said, is "What's it like living without your brother?"
Diesel named his daughter Pauline, now two years old, in honor of Walker.
Costar Michelle Rodriguez said the death of Walker and his absence is the reason why the eighth film in the franchise is darker than the rest.
"We didn't want to jump in and pretend that everything's OK after such a big loss," she said. "You kind of have to reinvent the thing and go dark before coming out of it."
Ludacris said the cast wants to honor Walker, who was more than just their costar.
"The last movie was for Paul, we love to say this movie is from Paul," Ludacris said. "So, I mean his presence is always going to be felt and at this point we're working hard and trying to make the best movie possible for him to continue on his legacy."