Haven and Eureka are filmed in Vancouver, Harper’s Island was filmed in Vancouver, five seasons of The X-Files were filmed in Vancouver as were the tangentially related series The Lone Gunmen and Millennium, Kingdom Hospital was filmed in Vancouver… And lots more shows, especially those set in the US northwest and northeast. (Apparently some shows are now being filmed in Toronto too, though glancing through the list on wiki I mostly recognize Canadian shows set in Canada, with a few exceptions like The Dresden Files and Kevin Hill)
How does this work for the actors, though? Most of them are US citizens. Do they need to get work visas? Where do they live while filming? How long does it take to film 13-24 episodes/year of a TV show, anyway?