How cast of '100 Days of Deception' trained for multilingual roles

TvN’s latest K-drama juggernaut "100 Days of Deception" is fast becoming the season's breakout standout. Headlined by Hallyu heavyweights Kim You-jung and GOT7’s Park Jin-young, the historical thriller achieved a series-high rating with its Sunday episode, hitting a 9.9 percent nationwide average and dominating its timeslot across all broadcast and cable networks.

Set against the backdrop of the Japanese colonial era, the espionage drama centers on Lee Ga-kyung (Kim), a street-smart thief pressed into intelligence work, and Sato Hideo (Park), an elite interpreter whose fate becomes bound to hers.

Beyond the historical stakes, the narrative demands a rare feat from its lead stars: commanding a multilingual script spanning Korean, Japanese and English.

To meet those demands, the cast underwent exhaustive preparation. In a recent release, tvN pulled back the curtain on the intense linguistic boot camp designed to have its actors bring their characters to life.

Director Yoo In-sik said the actors began preparing for their foreign-language performances as soon as they were cast, impressing him with their proficiency at the first script reading.

“Given the sheer volume of foreign-language dialogue and the many scenes in which they had to interact directly with Japanese, American and British actors, the cast continued practicing until the final day of filming — in some cases, for more than a year,” Yoo said.

“Japanese, English and Chinese language coaches worked with the actors one-on-one. Whenever a scene involved foreign-language dialogue, they were on set to check each line and provide guidance.”

Kim You-jung devoted about five months before filming to studying Japanese. Rather than simply memorizing her lines, she took three separate classes focused on grammar and conversation, as well as mastering the script. Despite her busy schedule, she attended two lessons almost every day, rarely taking a day off.

Her main focus was conveying the resourcefulness of Lee Ga-kyung, for whom Japanese and English are languages of survival.

To capture the character’s distinctive speech patterns and intonation, Kim first memorized her lines, then worked repeatedly to refine her delivery according to the emotions and circumstances of each scene. She paid particular attention to how Lee speaks English, given that the character picked up the language informally. Kim worked to convey the subtle impression that Lee was piecing together words she had learned by ear rather than speaking with fluency.

The Japanese-language coaching team praised Kim’s dedication, noting that she was the first cast member to begin lessons. When traveling overseas, she continued taking remote classes between scheduled activities.

“She was particularly good at internalizing the intonation and then making it her own,” the coaches said.

“Eventually, she could recognize even the slightest mistake in her own intonation. The sheer amount of practice she put in was remarkable. She was also adept at adapting her delivery to different people, much like Lee Ga-kyung herself.”

Park Jin-young likewise delivered his Japanese and English dialogue as elite interpreter Sato Hideo, also known as Kim Tae-woong. For Park, the key was relentless practice. Beginning in February, he repeatedly rehearsed the Japanese lines in his script until filming wrapped.

He also differentiated between American and British English to reflect the distinct backgrounds and characterizations of Lee Ga-kyung and Sato Hideo, adding another layer of detail to their performances.

The Japanese-language coaches noted that Park already had some basic Japanese skills compared with his fellow cast members. He could even write hiragana, which he used to transcribe his Japanese dialogue onto a large sketchbook as a memorization aid.

“He said he remembered his lines better when he paired them with movements, so he would rehearse the dialogue while physically acting out what he had in mind. It felt almost like watching someone prepare for a stage performance,” the coaches said.

To help Park develop the right intonation and speech patterns for his character, the team also asked several speakers to record reference dialogue.

Park listened so intently that the voices stayed in his head even after he lay down to sleep, they recalled. Despite the challenge of taking Japanese and English lessons back-to-back, he approached the preparation with enthusiasm throughout.

Kim Hyun-joo, who plays Yoo So-ran, was the only cast member to take on Chinese-language dialogue. Her character is Chinese but speaks Joseon language, the Korean spoken during the period. Because Yoo So-ran is not a professional language user, Kim prioritized a natural delivery over overly precise intonation.

With few historical resources documenting the linguistic characteristics of the era, Kim closely observed how her Chinese-language coach spoke Korean, paying attention to the coach’s intonation and speech patterns. She also studied the Korean pronunciation of various Chinese speakers to develop a delivery suited to her character.

For Kim, conveying each scene's tension and her character’s emotions convincingly was more important than achieving linguistic perfection. She therefore focused on delivering her lines in a way that felt authentic to the circumstances.

The coach who helped Kim with her Chinese dialogue explained that the training began with the basics of Mandarin pronunciation, particularly its four tones, which differ from Korean. Once Kim had learned the pitch contours and articulation, she gradually applied them to the script, starting slowly before working toward a more natural delivery.

“Kim paid close attention to the pronunciation and intonation of every line,” the coach said.

“What impressed me most was how she kept refining her delivery to make it sound natural, never overlooking even the smallest differences in pronunciation. She maintained her concentration through countless repetitions and continued practicing after class by listening to audio recordings I had made of her lines.”

The director and cast also worked to capture subtle differences in how each character spoke a given language. One example was Yoo Philip, played by Lee Moo-saeng, whose Japanese dialogue needed to sound distinctly like that of a foreigner.

The Japanese-language coaches recalled that Lee had memorized nearly all his Japanese lines by his first lesson in the winter of the previous year. Rather than focusing on conventional language instruction, the team showed him reference videos to help him reproduce the characteristics of Japanese spoken by English speakers.

Lee approached the role with the understanding that an American speaking Korean or Japanese would sound different from a native speaker. He worked to develop a distinct linguistic style for Yoo Philip, ensuring that the character’s profession as an international correspondent stationed overseas also came through in his speech.

Lee appeared to draw on a natural aptitude for languages, favoring concise, precise sentences complemented by facial expressions and gestures. He also watched films from the 1930s to study the speech patterns and physical mannerisms of people from the period, using those observations to flesh out his character.

Jin Seon-kyu, meanwhile, spent two months in Japan preparing for "100 Days of Deception," memorizing his script for seven hours a day. His commitment translated into a performance so convincing that it earned praise in Japan, with his natural pronunciation and intonation belied by the fact that he had never spoken Japanese before.

The Japanese-language coaches said Jin’s first lesson took place in October of the previous year. When they learned he planned to spend about two months in Kyoto, they worried he might pick up the local dialect. To help him avoid that, they found an actor in Kyoto who could coach him on his Japanese dialogue.

During his stay, Jin took Japanese conversation lessons in the mornings and rehearsed his scripted dialogue in the afternoons. He also attended remote sessions with the coaching team in Korea twice a week to review his lines.

He additionally had to learn dialogue in Korean spoken by Japanese characters — a considerable amount of material the coaches said was almost overwhelming.

“Even so, he handled it so well that all we could do was marvel at his efforts,” they said.

In closing, the Japanese-language coaching team emphasized that every cast member had memorized all 12 episodes’ worth of Japanese dialogue before filming began. While there were occasional mistakes in intonation, they said, actors rarely struggled to recall their lines on set.

“Even with language coaches, this level of preparation would not have been possible without the actors’ own dedication,” the team said, praising the cast’s commitment to the production.

This article is based on a tvN press release and was translated using AI and edited by Korea Herald staff. — Ed.