How to sneak on Gothic 1
Sneaking in Gothic is useful to enter houses at night without waking up the owners, or to avoid being seeing by walkbyers while you are attempting to become the new owner of someone else's goods without him knowing.
Press the X key to activate the sneaking mode.
Don't get caught: if someone sees you sneaking, you will need to give up on your criminal plans for a moment.
Also read the complete Gothic 1 guide.