In the summer of 1999, Heather Donahue, Michael C. Williams and Joshua Leonard were trapped in a strange sort of limbo. “The Blair Witch Project,” their debut feature, had exploded out of the Sundance Film Festival that January to become one of the most influential horror movies of the past 25 years — and, with a $35,000 budget, one of the most profitable independent films ever made. And yet Donahue was still puttering to her temp job in a 1984 Toyota Celica, before it broke down right underneath a billboard with her face on it. Williams’ boss kept asking him why he was still moving furniture in Westchester, N.Y., when he was on the cover of Newsweek. And Leonard found himself serving food to his agent at a catering gig days before he appeared on “The Tonight Show.”