Be careful about transforming into a spider if there are any guards around.

A mage character can also use their Swap Places special move to do this, though neither technique will work to let you get into Kistandalius' back room in Ars Magicana.

There is an exploit which will allow you to steal without any style:
If you drag and drop an item into an open container in your inventory it is not considered stealing. It will be seen as stealing when you remove items from that container, so make sure you are alone before doing so. At the start of the game Lanilor has a low weight chest in his house and later you can get sack of grain when you get to the farmlands; the lowest weight container in the game can be found in Verdistis, if you join the thieves' guild.