The Fast & Furious star majored in the subject in college, and in 2010 he starred in a TV special for America's National Geographic Channel called Shark Men.

Walker spent a week catching and tagging great white sharks off the coast of Mexico with Dr. Michael Domeier, and the marine life expert has now revealed his protege became a close friend and even sought his advice about quitting Hollywood to pursue his dream.

Domeier tells Entertainment Weekly, "He would call me in the middle of the night and say, 'Hey, I want to quit acting. I want to go back to school. Can you help me?' And I'd talk him off the ledge and go, 'Paul, that's a really bad idea. You can have much more of an influence on these topics as a celebrity. Let's work together. I can help you, give you talking points'...

"So we kept in touch over the years, even after we both stopped working on Shark Men. He kept calling me: 'Hey, let's do something again. When can I come with you again?'"

The pals reunited for another TV special called Spawn of Jaws last year (13), and they also began filming a sequel, Spawn of Jaws: The Birth, just three weeks before Walker died in a car crash.

Domeier adds, "Honestly, I really didn't want to keep working on the film. But after a few months went by, I decided, 'Okay, Paul would want us to finish it, let's finish it.'"

Spawn of Jaws: The Birth aired in America on Wednesday night (13Aug14), and the programme was introduced by Walker's Fast & Furious co-star Dwayne Johnson, who encouraged viewers to make a donation to the late actor's charity, Reach Out Worldwide.