In a japanese interview, Supernova’s Sungjae mentioned Changmin during his time in the police promotions unit

Translation:

-only Changmin-related parts-
Q. During your enlistment, the police agency’s poster which you shot together with TVXQ’s Choikang Changmin, Super Junior’s Siwon and Donghae became a hot topic amongst fans.

A. That’s right. As I was with singer sunbaes who have been active as singers for much longer than myself, I was ashamed and felt pressured as well. (smiles) As seniors, they gave me advice, and as our ages were close, I think of the time that we spent together as special and one that I can never forgot. It was really a fun time.

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Credits:

Wow! Korea,
Translated by @snxy,
Shared by TVXQ! Express