GReeeeN made their debut in 2007 as a group who did not show their faces. Other than the fact that the members are all registered dentists, there is little known about them. The song "Kiseki" was the most downloaded single in Japan which made it to the Guinness Book of Records and was the theme song for TBS drama series "ROOKIES".
Matsuzaka revealed that he didn't know what to expect at first when told about the movie offer but after meeting JIN and HIDE during a meal in early May, he became intrigued and was touched by their story. As he has to sing in the movie, Matsuzaka admitted that he was worried but JIN assured him that he'll be fine if he sings in an enjoyable way. As for Suda, he was impressed by how the members managed to juggle being dentists and musicians at the same time and is eager to express their story through the movie.
Sources: Nikkansports / Sanspo / Oricon