Three weeks ago, the film's writer-director Josh Waller attended a screening of the movie with Monteith, who had shot the film last September.
"He had expressed that he was kind of going through a rough time," Waller tells ET Canada, "but that he was feeling better about it, and then when I saw him at the screening he wrote me a really sweet email."
The actor had been about two months out of rehab for substance abuse and was reunited with girlfriend Lea Michele.
Waller tells PEOPLE he had no hesitation casting somebody who had a drug problem in such an intense role.
"He was like: I can do this character, I know this character, I was this character, I have lived elements of this. He said I was a troubled youth," says Waller. "He was so charismatic and passionate about the material it was just a combination of everything that made me think this guy was going to do it."
On Saturday, Monteith, 31, was found dead in a Vancouver hotel room of unknown causes.
Waller found out from Monteith's McCanick costar Tracy Toms.
"The bottom dropped out a little bit, and I kind of knew what [her text] was going to say, but I was just going to hope from the get-go that it was a hoax," says Waller. "It's horrible, but I haven't really processed it yet."
Fans of Monteith's may make donations in his name to three charities that were especially important to him: Project Limelight Society, which exposes youth living in Vancouver's Downtown Eastside to the arts; Virgin Unite, founded to support entrepreneurial efforts to better the world; and Chrysalis, which helps homeless and low-income individuals find employment.