On a November night in 2013, the action star was shooting a climatic scene where his character Dominic Toretto performs a sacrificial, “semi-kamikaze” stunt to save his friends. The maneuver leaves Dom lifeless. Paul Walker, who plays Brian O’Conner, rushes over to revive him. “These action films can very dangerous, no matter what anyone tells you,” says Diesel, noting that the scene was so real he envisioned his own death. “I started to think, ‘What would happen to Paul Walker if I died?’”

After the sequence wrapped, Diesel wanted to speak with Walker, so he went over to his buddy’s trailer. The two actors had appeared as onscreen partners for more than a decade, starting with the original 2001 “The Fast and the Furious,” in which Diesel portrayed the bad boy (a rogue street racer) to Walker’s buttoned-up cop. “If I do die, let them know what kind of brother I’ve been to you,” Diesel recalls telling Walker. Walker gave him a hug, and the two men parted. “I’ve played that over in my head countless times. That’s the last time I ever saw him,” says a teary-eyed Diesel, choking up.

Just a few days later, on Nov. 30, Walker perished, the passenger in a high-speed crash in a sports car in Valencia, Calif. The world mourned the 40-year-old actor, a James Dean for the millennial generation, and Universal implemented a four-month break in the production of “Furious 7,” as the film’s creative team scrambled to cobble together a new ending. “We grieved accordingly, and then at some point reality sank in,” says director James Wan. “We realized we still had to finish a movie."

Now, “Furious 7” is finally revved up for an April 3 debut, as one of the year’s most anticipated blockbusters. The stakes couldn’t be higher: “We never would have made the film if we didn’t think we could do justice to Paul,” says vice chairman of NBC Universal, Ron Meyer, who had to gently convince Diesel to stay aboard after Walker’s death. So devastated that he couldn’t leave his house for weeks, the actor says, “There was fear. Could I finish playing Dom with such a broken heart?” He finally decided to continue after a conversation with Meyer, who told him he could back out if he wanted, but offered him a reassuring note of confidence. Having seen the dailies, Meyer told him, there was magic in what had been shot so far.

Says Diesel: “I thought what Paul would really want me to do was finish it. So that’s what I did.”

The “Fast” saga, the studio’s largest franchise to date, boasts a worldwide box office of $2.4 billion, and is known for its escapist explosions. But reality looms over “Furious 7,” and fans are expected to flock to multiplexes to say goodbye to Walker in a similar fashion to the way 2008’s “The Dark Knight” doubled as a memorial service for Heath Ledger.

Paying homage to the loss of Walker in its marketing materials to promote the film, Universal created a billboard featuring a black-and-white two-shot of Diesel and Walker with the tagline: “One Last Ride.”

Diesel admits to having cried more in the past year than in his entire life. “I post a picture on Facebook, and I’m just bawling reading the comments,” he says. Walker’s passing not only affected him personally, it changed him as an actor. “It’s made for a more emotional performance,” he explains.

On his first day back on the set, Diesel had to shoot a scene in his trademark 1970 Dodge Charger, where Dominic is meant to challenge Jason Statham’s Deckard Shaw to a race. “I’m supposed to be in killer mode,” Diesel recalls. “I went through three boxes of tissues, and I felt so bad. I felt so embarrassed. I had always been the kind of actor that other actors respect. I was just failing so hard. My nose was running and my eyes were tearing. I had to walk off set and try to get all the fluids out. I couldn’t contain my emotion, and thus it became the toughest film I ever had to shoot.”

Even if Walker’s memory is at the forefront of “Fast 7,” Diesel has always propped up the franchise. “He lives, breathes and sh--s ‘Fast and Furious,’” says Ludacris, who plays Tej Parker in the series.

At a time when action stars are becoming an endangered species — just ask Chris Pine, Ryan Reynolds or Armie Hammer — Diesel, who headlines such testosterone-filled adventures as the “Riddick” and “XXX” series, is a throwback to the ’80s prototype of masculinity exemplified by Arnold Schwarzenegger, Bruce Willis and Sylvester Stallone. It’s not that he doesn’t have other shades to his career (he started as a writer-director of the 1997 Sundance indie “Strays,” based on his life as a bouncer, and before that, the 1995 Cannes short “Multi-Facial,” a sendup of the years that Hollywood rejected him for not looking like a conventional leading man). But he doesn’t mind that “Fast” has typecast him as a tough guy. “I saw Mel Gibson over the weekend, and he was telling me to direct,” says Diesel, who hopes to return to that job one day. “But the studio needs for the saga to continue.”

Diesel isn’t just the face of “Fast,” he’s also a producer and the guardian of the franchise. “I’m not the writer, but I’m the saga visionary,” he explains. “There’s nobody in the world that could sit down and tell you what each story is.” Universal relies on Diesel to help provide the structure of the stories, offer input on casting, and even select songs for the soundtrack. “He’s a really good barometer of what the characters might or might not do,” says Universal Pictures chair Donna Langley. “He keeps us honest.”



In 2001, he suggested that Michelle Rodriguez play his girlfriend in the first “Fast,” and came to her defense when the original script had her jump into Walker’s bed, too. “I didn’t want to be a slut,” Rodriguez recalls. “I felt like a lot of Latinas in Hollywood were doing that, and that’s not how I wanted to be viewed by millions of people around the world.”



The “Fast and Furious” movies are an oddity in Hollywood. While most sequels are adapted from comic books, TV shows or videogames, “Fast” is perhaps the biggest franchise since “Star Wars” that’s based on an original idea. It’s also the rare blockbuster made up of a cast that actually looks as diverse as its audience: Rodriguez (Latina), rappers-turned-actors Tyrese Gibson and Ludacris (African-American), the Rock (of Samoan ancestry), Sung Kang (Korean-American), Gal Gadot (Israeli) and Jordana Brewster (Brazilian-American), with the pan-ethnic Diesel as the leader.



According to exit polling from Universal, the U.S. audience for the 2013 opening weekend of “Furious 6” was 33% Hispanic, 22% African-American and 13% Asian-American. Caucasians made up only 29% of viewers, meaning twice as many tickets were sold to non-white moviegoers.



As the series hits its seventh installment, it shows no signs of slowing down — the sixth film was the most successful so far (with a global box office of nearly $800 million). “The franchise will continue to grow,” Langley says. “We’ll keep making them as long as people want to see them.”



Diesel has ideas for as many as 10 films, which he sometimes bounces off his groupies. And he’s got plenty of them to poll. He’s the fourth most-popular celebrity on Facebook, with 90 million fans (ahead of Justin Bieber, Taylor Swift and President Obama). He recently welcomed his third child, Pauline (named after Walker), with partner Paloma Jimenez, but when he speaks about “Furious 7,” he sounds like a proud dad of the movie, given to unabashed hyperbole: “Universal is going to have the biggest movie in history,” Diesel says. “It will probably win best picture at the Oscars — unless the Oscars don’t want to be relevant.” Awards predictions notwithstanding, “Furious 7” will make history.



Although other projects have used face-grafting imaging to fill in an actor’s performance after death (most notably Brandon Lee in “The Crow” and Oliver Reed in “Gladiator”), the level of sophistication in creating the computerized Walker is groundbreaking. The project enlisted the help of three stands-in: Walker’s two brothers, Caleb and Cody, and a third actor, John Brotherton. Peter Jackson’s Weta Studios (which created Gollum in “Lord of the Rings”) added Walker’s face in post-production, a process that’s been kept a closely guarded secret until now. “It’s a game changer when people are capable of manipulating your body after you’re gone,” Rodriguez says. “You’re talking about a whole new conversation that needs to be had in Hollywood. A person’s brother, mother or sister is not going to have the same integrity or taste as you.”



On one hand, the support from Walker’s family was emotionally helpful. “It was endearing that his brothers were there to assist with his legacy,” Diesel says. At the same time, the actor found himself in the surreal position of coming to work, pretending that his friend was still alive. “The irony is, I’ve been acting since I was 7 years old,” say Diesel, whose stepfather was a theater teacher. “No one in the world could teach you how to act across from someone your heart is mourning for. That’s like asking someone who has lost a parent to replay it again and again, and pretend there’s not a void there.”



Adds producer Neal Moritz: “It was really hard. Vin would flash back to all the times Paul was there.” Diesel’s sister Samantha Vincent, an executive producer on the past four “Fast” movies, witnessed his pain: “Paul was probably one of the only people in Vin’s life who really understood Vin, because they shared this ride together. I don’t think his death is something he’ll ever truly get over.”