Kim has been turning heads at home since the 2011 TV drama musical , in which he stars alongside some of the top names in K-pop including miss A's Suzy and 2PM's Taecyeon. In early 2012 he played a king in the explosively popular TV drama , which attained a 42.3 per cent viewership and made him a household name.

In a sign of just how popular he is, last year the heartthrob set a new record for endorsements, acting as the face of 17 products simultaneously. (The previous record holder was 2010 Olympic figure skating champion Kim Yu-na with 15.)

might seem like an odd choice for an actor known for his well-groomed image and reserved personality. Kim plays an elite North Korean spy who, after being dispatched to the South, disguises himself as a village idiot. With tangled, overgrown hair and a distinctly unfashionable green tracksuit, he suffers abuse and makes a fool of himself on a daily basis, while surreptitiously taking detailed notes on the mundane everyday habits of his neighbours. Eventually, he is joined by two other spies who pose as a rock musician and a high school student respectively. Then one day they receive a distressing order from their superiors in the North.

The sophomore feature by up-and-coming director Jang Cheol-soo ( ), is based on , a popular 2010 webtoon series of which Kim professes to be a fan. "I had read the original webtoon, and became totally engrossed in the story," the star says. "While reading it, I laughed a lot, and then found myself crying. That made me want to take on the challenge of playing this contradictory character."

embraces a wide range of moods, from broad comedy and sentimentality to a large-scale action sequence in the final reel. Although the film has received mixed reviews, Kim is recognised by critics as not just a pretty face but an actor of real talent.

Nonetheless he remains modest about his achievements. "It's quite difficult for me to depict a character with whom I share nothing in common, and to express thoughts and emotions I've never experienced before. But this attempt makes me feel fulfilled as an actor."

Although is his first lead role, Kim also left an impression as part of the ensemble cast that powered last year's runaway hit . The character he plays, a young burglar who harbours a crush on Gianna Jun Ji-hyun, has limited screen time but he's likely a key contributor to that film's record-breaking 13 million admissions (US$83 million) box office score as well.

On working with the acclaimed cast, which included Hong Kong stars Simon Yam Tat-wah, Lee Sinje, and Derek Tsang Kwok-cheung, Kim says: "That was my first experience acting in film, and what impressed me more than anything else was the level of concentration shown by these great actors. Shooting in Macau, everyone was exhausted and sweating from the heat but their concentration never wavered."

At the moment, with images of his face plastered around Seoul, he is South Korea's hottest celebrity. But the actor is evasive about his next screen appearance. "I haven't decided on a particular genre or type of character for my next role. But I do want to keep challenging myself with characters that can attract the attention of the public."

Meanwhile, he has returned to school, to continue pursuing his acting degree at Chung-Ang University in Seoul.

Secretly Greatly