As unthinkable as it might sound to legions of swooning fans of Descendants Of The Sun across Asia, Captain Yoo Si Jin did not exist in an early screenplay for the K-romance, and actor Song Joong Ki was not the top candidate for the role of the dashing soldier.

In fact, the biggest success story in South Korean entertainment this year is also an extremely unlikely one: What were the chances of a film company going into television production for the first time with a 13 billion won (S$15 million) pre-produced drama, and getting everything right?

Not good, to say the least. "Everyone said we were crazy," said Mr Kim Woo Taek, chief executive of New Entertainment World, in an interview with JoongAng Ilbo, which was published last Friday. "But based on our confidence in the story's content, we started planning," he told the Korean daily.

Mr Kim - who has executive-produced popular Korean movies including the 2006 actioner The Host and the 2008 thriller The Chaser - said he picked up Descendants by chance. Early last year, he heard that a friend in the movie industry was making a TV show, and he decided to produce it because "it contains the universal values of humanity, love, and sacrifice".

Crucially, screenwriter Kim Eun Sook - the creator of successful K-dramas including The Inheritors (2013), Secret Garden (2010) and Lovers In Paris (2004) - was then brought on board to rework writer Kim Won Suk's script. She beefed up the love story by introducing romantic military characters including Captain Yoo (Song), who pursues a surgeon (actress Song Hye Kyo) in between dangerous missions.

"Kim Won Suk's original screenplay tells the story of doctors who are active at disaster sites," Mr Kim Woo Taek told JoongAng Ilbo. "After we became responsible for the production, screenwriter Kim Eun Sook did a rewrite and increased the love plot. She added military characters, overseas disasters and other elements to flesh out the story. When I read the script till episode four, it seemed good."

Descendants is the first K-drama to air simultaneously in South Korea and China, where it has been a big hit, with 1.1 billion views on online video provider iQiyi. It is also a rare Korean TV show to have been produced entirely before the premiere.

Mr Kim said: "We're a newcomer to the TV market, and I wanted to enter it in a new way." Because a drama now has to be cleared in its entirety for streaming in China, "producing it 100 per cent in advance was inevitable".

For example, in episode one, Captain Yoo has a politically sensitive scuffle with some North Korean soldiers - a scene that is cut in the China version.

For the role of Captain Yoo, there were "other superstar candidates", Mr Kim told JoongAng Ilbo. They hesitated because Descendants had to be pre-produced and filmed overseas.

But unlike them, Song Joong Ki, who was about to complete military service, said: "Why wouldn't I act in such a good drama?"

The actor "actively" pursued the role, Mr Kim said. The boyish Song "has a relatively weak image", which is opposite to the manly Captain Yoo, but it leaves more to viewers' imagination, the executive producer said.

"During the costume test, I saw Song Joong Ki in uniform and knew it would be a success."

Noting that Song Hye Kyo is popular in the Chinese-speaking world, Mr Kim added: "The casting of the two is very successful."

As the pace of Descendants hots up, so has conjecture about the ending.

"My wife thinks someone will die," Mr Kim said. "She's always asking me who will die, but I haven't told her." Mrs Kim, like everyone else, might have to wait for the finale, which will air in Korea and China on April 14 (and in Singapore, on video app Viu, by April 15).