The Walking Dead has sold 8.5 million episodes to date, Telltale has claimed.

iOS versions of the game accounted for 25 per cent of total sales, explained Telltale CEO Dan Connors, with the average user spending "about $16". Each episode in the five-part series typically retails for $4.99.

"November and December [2012] were our biggest months," Connors told the Wall Street Journal. "It was the culmination of the product, the game of the year awards, the retail push, and it still has a ton of head room in it. Apple has featured us a bunch of times for each of the episodes, and it's in Apple's Hall of Fame (for iPhone and iPad apps)."

The first episode of the game was made available for free throughout the Christmas period, a promotion Telltale suggests successfully brought "more people into the product", and allowed the developer to experience a "very, very strong conversion rate".

"This thing is really the best execution of the episodic model," added Connors. "What we've done is have this online buzz going from April to November and then have this finale to top it off and have that be the biggest event that propels it into the mass market.

"It allowed us to capture all that early adopter audience but also gives us the momentum to build a product that can reach out to the mass. Instead of paying a bunch of money to drive a bunch of installs, we have all this momentum and recognition that comes from all the work we've done over the five months leading up to the Christmas launch. Then the finale happens, and bam, it's picked up by a much wider spread."

The Walking Dead has already picked up multiple GOTY awards by critics, including Game of the Year at last year's VGAs.

The game landed at the runner-up spot in our Game of the Year awards.

Source: Wall Street Journal | Behind the Hit iPhone Game "The Walking Dead"