In Dead Space 3, you can take on the Necromorph plague in pure spine chilling isolation, without any AI character following you, or you can co-op it with a friend, and be introduced to Carver.
Carver has his own story, his own issues with going through dementia and Markers, so the player will experience a completely different side to Dead Space 3 than he would just playing alone. Check out the video from IGN below for a behind-the-scenes look at all motion-capturing that went into creating Carver. If nothing else it shows just how awkward it is trying to act while being motion captured in a silent empty warehouse as opposed to on a fully designed set. It looks like a tough job requiring a lot of imagination from both the actors and director.