Opponents and civilians all have a Metal Gear Solid -esque visual field and the game allows you to look at one person's field of vision at a time. Elevation, lighting and obstructions affect what a character can see, and if someone is intent on something, their field narrows to focus on it. Overall the system is very convincing, and allows plenty of scope for hiding behind things, lurking in shadows and keeping low to minimise your profile. Your own characters aren't restricted by these factors - you can see the whole level and all its inhabitants. This lack of a fog of war may not be very realistic, but it stresses the puzzle and planning elements of the game.
Level design is excellent, if challenging.
It's not unusual to have to try a particular plan time and time again, and sometimes all that makes the difference is a tiny matter of timing or positioning. Sometimes it can be maddening, but I always found the game good enough to keep me trying again and again, and the challenge means that success is hugely satisfying. The AI of opponents is very impressive. Every character has a number of traits at different values, like accuracy, bravery, cleverness, laziness and alcohol resistance, which create mini-personalities for the different character types. While some are quick on the uptake and persistent, others are dimwitted and likely to give up a search after only a brief time.