"He opened up quite a bit," Waller tells PEOPLE of their meeting for the movie that shot last September. "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.' "
Less than a year later, there were questions of whether Monteith's past had caught up with him after the Glee star, 31, was found dead in a Vancouver hotel room.
A cause of death was pending the results of Monday's autopsy and toxicology tests. "We genuinely don’t know," says coroner spokeswoman Barbara McLintock.
The Canadian native completed a month-long stint in a treatment facility for substance addiction in April and had been open about his lifelong struggles with substance abuse.
Appearing on Inside the Actor's Studio last year, Monteith said that his problems with drugs and alcohol began when he "was about 12."
He attended 16 different schools, including some that had alternative programs for "kids that are problematic," before dropping out at the age of 16.
By that stage his drug use had become "a serious problem," and, as Monteith revealed in a 2011 interview in Parade he was using "anything and everything, as much as possible."
Worried that he was going to die, Monteith's mother and a group of friends staged an intervention when he was 19. "I did the stint but then went back to doing exactly what I left off doing," he said.
The turning point came when he was caught stealing "a significant amount of money" from a family member. Monteith was given an ultimatum: Quit drinking and taking drugs or go to jail.
"I changed my whole life," he admitted. "I moved to another city to live with a friend of my mother's who was recovering from addictions and I decided I was going to start trying to learn how to act."
After moving to LA and scoring the role as high school student Finn Hudson on Glee in 2009, Monteith made a concerted effort to remain sober.
Carin Baer / FOX
When asked on Canadian talk show George Stroumboulopoulos Tonight whether he struggled with sobriety in LA, he answered, "No. I just redouble my efforts on what I'm doing."
Monteith leaned heavily on his girlfriend and costar Lea Michele, who stood by the man she called her "best friend" and was waiting for him when he was released from rehab.
It's still unclear what happened after Monteith arrived in Canada and checked into the hotel on July 6. (Michele remained in Los Angeles.) He and his manager dined on Thursday night with Maureen Webb, the co-founder of Project Limelight Society, who said the actor "looked so good, so healthy."
Police say that the next night, Monteith and three unidentified people left his hotel room and that the actor returned alone at about 2:15 a.m. Saturday. A hotel staff person found his body shortly after noon when Monteith didn't check out.
When he talked to Parade in 2011, Monteith said that while he felt he could resist the temptations of Hollywood, "I can’t completely rule out any of that happening."
"But," he added, "I can't un-live the life I've lived."
Reporting by MIA McNIECE
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.