Sol Kyung-gu (Yonhap)

(From left) Sol Kyung-gu, Kim Nam-gil, Seolhyun, Oh Dal-soo and director Won Shin-yun pose for a photo at a press conference in Apgujeong-dong, Seoul, Tuesday. (Yonhap)

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 Tuesday.“It’s not the same now that I’m older.”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 (“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 Joker in the 2008 film “The Dark Knight,” on his bedroom wall.“The more I looked at the poster, the more I saw that 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 girl group AOA member. “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)