Lee Kwang Soo and Kim Jong Kook showed off their bromance on the set of 'Sound of Heart'!

SEE ALSO: Kim Jong Kook to be the MC on the new hip-hop survival program 'Target: Billboard - KILL BILL'

On December 30, Lee Kwang Soo's agency released behind-the-scene photos from an episode called 'Twins,' where Kim Jong Kook played twin cousins of Lee Kwang Soo's character. The singer portrayed two characters well, earning praise from the staff. 

The two friends look friendly yet serious while practicing their lines and taking selfies!