James Marsters often speaks admiringly of Steve Tartalia, his stunt double on “Buffy” and “Angel,” during his Q&As at different conventions. Steve worked for many years in Hong Kong doing action films and then, as of season 4 of “Buffy,” became the side of Spike that faced off with the Slayer, Glory, her minions and all the baddies that showed up on “Angel” season 5. We sat down with him last week to find out how it all had come about, and this is what he told us.