Miso DexterSocial TV platform Miso is launching a new feature today that adds original synchronized content to TV episodes watched on a DirecTV set-top box.

Basically, when users check in using Miso’s iPhone app, they’ll get push notifications asking them trivia questions, identifying music that’s currently playing and sending interesting facts about the episode  — all in real-time.

“As you watch TV, the mobile device synchronizes with the TV, and we can deliver something interesting at a specific moment by working with the content providers,” said Miso CEO Somrat Niyogi.

The first show to get this new layer of social interaction is Showtime’s serial killer drama Dexter, which kicks off its sixth season premiere Oct. 2. Viewers that check in using the iPhone app will have a chance to cast their vote in polls asking “Who is Dexter’s next victim?”, share snarky dialog from Dexter’s mouthy sister Debra, receive alerts when their friends tune in and more.

Admittedly, Miso’s new social feature has the potential to add a lot of depth to TV shows with a strong following. Shows like Doctor Who and Chuck, which fans routinely watch multiple times, would definitely benefit from Pop-up Video-style notifications. The first viewing might be uninterrupted, but the second would add a level of value that’s currently not available when watching an episode alone.

“We’re sort of testing this out with a big show that people already engage with,” Niyogi said, adding that the new feature presents plenty of new possibilities for content providers to transform what’s usually a passive activity (watching TV) into highly social one.

Users must be DirecTV subscribers and use the Miso iPhone app over their home Internet connection for the features to work, but Niyogi said the company is working on partnerships with additional cable providers like Comcast, Dish TV and AT&T’s Uverse.

Miso has partnerships with several major television networks, including Fox, Showtime, NBC, TNT, Starz, QVC and Comedy Central.

Founded in March 2010, the San Francisco, Calif.-based startup has raised an undisclosed amount of seed funding from Google Ventures, Keith Rabois, YouTube cofounder Jawed Karim, and early Google employees Georges Harik, Richard Chen, Thomas Korte, and Kurt Abrahamson. It faces competition in the marketplace from similar services like GetGlue, SnappyTV and Foursquare.