Sol Kyung-gu has undergone another drastic transformation in the upcoming thriller “Memoir of a Murderer.”

The 50-year-old veteran actor, who was in Cannes in May with “The Merciless,” shed several kilograms to play a “retired” serial killer in his late 50s to early 60s, suffering from Alzheimer’s disease. “My memory has forgotten but my hand remembers murder, like a habit,” says the movie’s tagline.

“When I was in ‘Oasis’ and ‘Silmido,’ I was young, so it was easier to lose weight,” Sol said of his other works that involved extreme physical transformations at a press conference in Seoul on Tuesday.

“It’s not the same now that I’m older.”

With his face twitching and emaciated, the film’s trailer shows a frightening Sol who struggles between memory, instinct and his mental condition.

The film is based on the 2013 best-selling novel by Kim Young-ha of the same title.

Director Won Shin-yun, who was behind “The Suspect” (2013), confessed that he had read the novel in 40 minutes in one sitting, and right away decided to make a film adaptation.

“It’s fast-paced, with a whirlwind plot, and a sense of humor that accompanies the suspense,” said Won. “It wouldn’t have been right not to make it into a film.”

The novel tells the story of an elderly veterinarian, Byung-soo, who has forgotten his serial killer past. He encounters Tae-joo, a cop, and is instinctively able to recognize him as a killer. The basic premise is the same in the film, but several adjustments have been made, director Won said.

“There are situations, emotions and characters that are different.”

Kim Nam-gil plays the role of Tae-joo, whose chilling manner makes him another likely suspect of multiple murders in the film.

Kim said he prepared for his role with a poster of the late Australian actor Heath Ledger, who played the Joker in the 2008 film “The Dark Knight,” on his bedroom wall.

“The more I looked at the poster, the more I saw that the Joker’s expression was full of many emotions -- sadness, fear, regret. ... I tried to maximize my violent tendencies,” Kim explained.

K-pop songstress-turned-actress Seolhyun, who made her film debut in the 2015 “Gangnam Blues,” stars as Byung-soo’s unsuspecting daughter. She cares for her ill father and slowly comes to fear his identity.

“I have dieted a lot,” said the member of AOA. “And I know that if you don’t have carbohydrates in your system, you can’t think. But (Sol) was so good in all his scenes. I have much respect for him,” she said.

The film hits local theaters in September.

By Rumy Doo (doo@heraldcorp.com)