A snippet from a new preview at GamingHeaven (thanks to InfiniteSims for the tip):
In Sims 3 it really appears that the whole environment is at your disposal and you can literally go anywhere you want and interact with anything you see on some level. The AI deserves special mention because it seems that the Sims are now perfectly capable of looking after themselves when you aren’t in direct control of them. I witnessed my creation going to get something to eat, watching the television, taking a bathroom break and even having a shower, without any interaction from myself. Obviously as discussed earlier, their personalities will affect what they want to do so instead of just random actions in the last titles, this time around the Sims really do appear as if they are living out their lives as they (or you) see fit.
