Vin Diesel's latest movie, The Last Witch Hunter , wasn't just a passion project, it was ultimately a way for the star to deal with the tragic passing of his Fast & Furious co-star and longtime friend, Paul Walker.The story follows Diesel as an immortal witch hunter named Kaulder, who has been cursed to never meet his loved ones in the afterlife, reports Entertainment Weekly “Death was a huge theme last year,” Diesel said to the Associated Press . “There was something therapeutic about playing this role after going through [Walker’s death].”He went on to say, “The idea that someone immortal could be so melancholy. 2014 was a tough year for me in some ways — a lot of ways. Playing a character that was masking his sorrow — his loss — was very appropriate for the time, because that’s what I was doing in real life with the post-Furious 7.”And how does he feel about continuing the series that made him so successful sans Walker?"He [Walker] took a lot of pride in this franchise and he'd be smiling down at the idea of us actually doing what we talked about," Diesel said. "Which is going to the tenth chapter."