You can't also wave your flashlight - the light beam will alarm nearby NPCs (they can call for police). Doors, cabinets, drawers and other containers should be opened without making any noises by pressing the interaction button - otherwise, a nearby NPC can hear you and run towards the source of the sound.
Crouching also allows you to lean out from covers - use this option to see what is behind a corner without a risk of getting spotted. This way of moving is also useful when you have to walk next to a short fence or under a window - an NPC on the other side of those obstacles won't notice you. Your character crouches - you move slower, make less noise, can rob houses while its residents are inside. The stealth mechanic is rather basic and simple - you can activate it by pressing one button.
This chapter focuses on the stealth mechanic - read it to know how to use it to your advantage. This, combined with a rather poor tutorial, can cause players a few problems, especially when you want to rob a house that isn't empty. Sadly, the Stealth mechanic doesn't work very well. Stealth is one of the most important gameplay mechanics in Thief Simulator.