According to an official from Group Eight, the production house in charge of the drama, Kim Hyun Joong was selected to play Ji Hoo, because he was deemed suited for the character. SS501 member Kim Hyun Joong was cast as the F4 member Hanazawa Rui in the Korean version of Hana Yori Dango, also known as Boys Before Flowers, which is scheduled to be broadcast via KBS 2TV in the first half of 2009 with a total of 25 episodes. They performed their Goodbye Stage on Music Bank on June 6th, 2008, to return to Japanese activities. After success with their single "Deja Vu", they began to promote their second single, "A Song Calling For You". The title track was the first song to be promoted, and they started their comeback on music channel M.Net's M Countdown.
In 2008, SS501 returned to Korea with their single Deja Vu released on March 8, 2008.
A full album was released later in the year. The single debuted at 5 on the Oricon chart. The group had a Japanese single "Kokoro", which had multiple versions, including one with all members and five others featuring the members individually. In Korea, their fan club members number around 41,700. A Japanese fanclub has been established with the name of "Triple S Japan" it officially debuted on March 25, 2007, when SS501 will had first official meeting with the group. Later in 2007, the group debuted in the Japanese market. On their 600th day (January 28, 2007), they won the Mutizen award from SBS Inki Gayo for their song "Four Chance". As well as promoting the album on various variety and music shows, they also filmed a show on MNet called SOS. Singles from the album included "Unlock" and "Four Chance". In late 2006, they returned to Korea in order to promote their first full album, which was released on November 10. In mid-2006, they held their first concert, "Step Up Concert" in Osaka, Japan. The hiatus was due to Heo Young Saeng's throat condition which required surgery, thus resulting in a need for time to fully recover. SS501 debuted in 2005 with their first single "Warning" and a second single "Snow Prince" released in late 2005.The group was inactive in Korea for most of 2006, although they had a fan meeting in Japan in April.