I could have sworn in an interview they said it was RE5 style where you had an AI partner at nearly all times, and that if you played alone it just meant they had some parts where he would go ahead of you to solve a puzzle you couldn't do alone.
That's pretty much exactly NOT what they're doing.
http://penny-arcade.com/report/editorial-article/dead-space-3-added-co-op-the-expensive-way-and-boy-was-that-money-worth-it
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JonthePenguin
http://www.backloggery.com/main.php?user=erunion