2008-2009: Musical theatre

On 9 May 2008, Yesung was sent to the hospital after he injured his leg during the 70 km 24-Hour Marathon of Hope in which he and band mates Shindong, Sungmin, Leeteuk, Eunhyuk and Kangin participated. The event was in collaboration with the SBS charity program "Hope TV 24" and the money raised was donated to a Mongolian school in South Korea which was at risk of closing down. Due to the injury, Yesung was unable to complete the marathon, but appeared on stage on crutches with his fellow members. Also on 8 August, Yesung was sent to the hospital after falling from a 1.5m stage while rehearsing at the KBS music program, Music Bank. The injury aggravated old injuries from the neck and waist and was hospitalised for two days.

He sang ballad track "Love Really Hurts" for the original soundtrack of television drama Tazza, which aired from 16 September to 25 November 2008. In 2009, Yesung made his musical theatre debut in Namhansanseong (lit. South Korean Mountain Fortress), which is based on the novel of the same name by Kim Hoon, which is based on the historial incident of Byeongja Horan, at the Namhansanseong in Gyeonggi-do. But the musical focuses on the lives of common people and their spirit of survival during harsh situations. Yesung played villain "Jung Myung-soo", a servant-turned-interpreter who feels sad from being betrayed by his country, from 9 October to 4 November at the Seongnam Arts Center Opera House. On 6 November, he made a surprise appearance on KBS' Music Bank with label mates SHINee in place of Jonghyun, who was recovering from influenza, in a performance of "Ring Ding Dong".