Dead Island's success was all due to that stunning trailer, right? Well, according to Guido Eickmeyer, development director and executive producer of games at publisher Deep Silver, the resort-vacation zombie simulator sold 4 million units because of its cooperative gameplay.

"This is why we got all the good reviews from users. This is why we got high user ratings, because it is the most exciting co-op experience that is on the market," Eickmeyer said.

Despite investing less than $25 million dollars in the project – low for a major triple-A video game – Dead Island has been played for 56,907,547 hours in co-op, which is approximately 6,500 years total.

After the success of the trailer, there were talks of forming the game around that, but there simply weren't the resources. Dead Island showed strong potential in its co-op, so the focus shifted to that and that game was a huge hit. Dead Island: Riptide is another "huge co-op experience" being revealed this week at Gamescom.

This article was originally published on Joystiq.