The Kill Bill star plays a reincarnated medieval knight in the film, his last completed project before he died from sexual misadventure in a Thai hotel room in 2009.
And, as Sampson, who wrote, directed and stars in the movie, prepares to offer the finished cut to studios, he still can't believe Carradine's ad-lib at the end of the film.
He tells WENN, "On the last moment of the movie, we're walking off together, arm-in-arm, and the dialogue is like, 'Well, old friend, I'll see you in the next lifetime', and he goes, 'Old friends, old soulmates...' That's the last line.
"I get goosebumps every time I see it... He was dead a year later. It's the last thing he says in a film."
And Sampson insists that's not the only eerie thing about his film - it features a sex scene which apes the way Carradine may have died.
He adds, "(Actor) Norman Reedus' character is getting a b**w-job in the movie and he's choking himself with a belt - like the way David died."