• IU and Lee Joon-Gi star in the SBS drama 'Scarlet Heart: Ryeo.'

IU and Lee Joon-Gi star in the SBS drama 'Scarlet Heart: Ryeo.' (Photo : YouTube/Mari)

Lee Joon-Gi and IU's kissing scenes in SBS's "Scarlet Heart: Ryeo" has been a much-talked-about topic. In the ongoing historical drama, the two Hallyu stars portray 4th Prince Wang So and Hae Soo, respectively.

On Sept. 30, Friday, SBS shared a sweet behind-the-scene video of Lee and IU during the filming of the ninth episode of "Scarlet Heart: Ryeo" through Naver TV Cast. In the video, the 34-year-old actor is seen handling the kissing scene professionally, even helping IU find the right angle.

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The 23-year-old singer-turned-actress laughs uncontrollably when Lee eagerly advises in between takes that she should not pull her lips away. All through the video, the two lead actors are seen lightening up the mood on set, as they chat and joke around.

With the filming day falling on IU's birthday, the entire cast and staff members surrounded IU as they sing her a birthday song and present her with a cake. Lee then relays his best wishes to his co-star through a video message.

"I think of that kiss as my birthday present to you," Lee wittily said, as translated by Soompi. "It was a gift given against your will and that only I liked."

The ninth episode of SBS's "Scarlet Heart: Ryeo," which aired on Sept. 20, saw the emotional lip-locking scene between the 4th Prince Wang So and Hae Soo. The said episode recorded a national TV rating of 7.9 percent.

In a recent interview, the "Scholar Who Walks the Night" actor detailed the fourth prince's real feelings for the heroine. In a statement obtained by The Kpop Herald, Lee revealed that Wang So's feelings for Hae Soo will turn out to be increasingly proclaimed in the upcoming episodes.

"She is the only person that saw him for what he truly was," said Lee, referring to his character's relationship with Hae Soo. "He's also most honest when he's around her. She's somebody that he wants to protect, and he feels he is being protected by her as well."

Lee also teased some exciting details about what viewers should look out for from the fourth prince. From being the most disregarded prince, 4th Prince Wang So unexpectedly becomes the center of attention.

According to Lee, Wang So will say goodbye to his reluctant and introverted said and will now show a more confident and stronger character. In the upcoming episodes, the fourth prince is expected to tackle romance, jealousy, betrayal and more.

Check out the behind-the-scene clip below: