Siwan celebrated his birthday on the set of 'Run On' with Shin Se Kyung.

In the making-of video for episode 9 of the JTBC series, Siwan and Shin Se Kyung go over their scenes for the drama, including Ki Sun Kyum (Siwan) nursing Oh Mi Joo (Shin Se Kyung) back to health and dining in a cafeteria together. Because Siwan's birthday fell on the same day of filming, Shin Se Kyung and the drama surprised the actor for his 33rd birthday with a birthday cake.

After thanking everyone and blowing out his birthday candles, Siwan says, "I thought I was late to set because I saw all of you together."

Check out the behind-the-scenes video above!