In Season 1 of Arrow, I've noticed an unusual number of appearances of actors that are well-known for their roles in Sci-Fi shows.
- John Barrowman, known for his role as Captain Jack Harkness from Doctor Who and Torchwood
- Paul Blackthorne, known for Harry Dresden in Dresden Files
- Alex Kingston, known for River Song in Doctor Who
- Seth Gabel, known for Lincoln Lee in Fringe
- James Callis, known for Gaius Baltar in the new Battlestar Galactica
And likely more. (I've not finished watching Season 1 yet!)
Is there an explanation for this beyond just the "brand equity" these actors bring to the show? I normally don't recognize this many in show that's not that kind of science fiction. None of these actors are known for comic-related roles.
(I'm not familiar with all the workings of show business, but my hunch would be something like these shows sharing production staff, or someone involved in the making of the show being a "fan" of those shows, or friends with the actors. In a similar question I had about guest appearances, there was a surprising promotional explanation.)