Don't get the animated Dead Space movies and think that's what Dead Space is. Buy the first game and then you'll see what Dead Space is.
As for Dead Space 3... don't worry, in single player mode it'll be the same Dead Space we know and love. Isaac will be very much alone with no AI character tagging along.
Basically the game changes when you play with another character, both in style as well as the events and interactions you have with the game, essentially making Dead Space 3 two different experiences in one.
It'll have the traditional Dead Space feel and then it'll have the feel of what Resident Evil has turned into now. I think Visceral has done a really good thing with this in balancing out the two different styles of games different groups like to play and putting it all in the one game... cause believe it or not, but there are people who actually prefer the more action orientated thing rather than the mysterious tension building side of Dead Space that the rest of us love.