The Brooklyn actor who played Johnny Cakes — the gay-fireman lover of a mob capo on "The Sopranos" — killed himself in a holiday tragedy that has stunned family and friends.

The front door to John Costelloe's Sunset Park home was still sealed with police stickers yesterday, more than a week after the rugged 47-year-old actor committed suicide.

Costelloe, a former FDNY firefighter, shot himself in the head in his basement bedroom on Dec. 16, cops and pals said.