When I started working on BATTLESTAR GALACTICA, I kept trying to place Aaron Douglas (Chief Tyrol). After much consternation, I finally just asked him, and remembered that he played a cop in one of my first-season SMALLVILLE episodes, "Obscura."
Then there's Lorena Gale, who played Priestess Elosha in BSG -- until she planted her tootsies on an especially nasty landmine, that is. Lorena played Lex Luthor's shrink for several eps back in my SMALLVILLE days, where she met an equally nasty, albeit off-camera, demise (car wreck, possibly/probably engineered by that magnificent bastard Lionel Luthor).
And there's more! Robert Wisden, who played Chloe Sullivan's father on SMALLVILLE until he got blowed up good, was "Wallace Gray" on a first season BATTLESTAR ep. For once, a BSG player who DIDN'T die (yet).
And don't forget poor Kat Katraine (Luciana Carro), who left this mortal coil in this season's BSG episode "The Passage." L.C. had two guest shots on SMALLVILLE. Even the Chief's squeeze Cally (Nicki Clyne) played a waitress in Lana Lang's cafe back in SMALLVILLE season one.
It's just like ol' Walt Disney said, "it's a small(ville) world after all!"