You play Dishonored from a first person perspective, never leaving Corvo's body. The game is mostly an action game, having you dispatch enemies as you move towards a certain objective, but there are a few light RPG elements as well, as you are able to upgrade your gear, and quite frequently are given the choice of how to deal with different situations. Throughout most of the game, you'll be carrying two weapons: In your right hand is your sword, and in your left hand is whatever weapon you manually equip. For some reason, they've decided that since the sword is your "main" weapon, it should be used with the left mouse button, despite being in your right hand; consequently, the weapon in your left hand is used with the right mouse button. It will take a while to get the hang of this, and even well into the game, I found myself pausing to check that I had the correct button when executing an attack.
You'll encounter weapons such as a pistol, crossbow with various types of ammunition, as well as grenades and shrapnel mines. In addition, the left hand is also used for equipping your magical powers. You block attacks with your sword by using the ctrl button on your keyboard, and if you time things right, attacking right afterwards will turn out a really cool kill animation. It works rather well, and the sword play especially is a lot of fun.
That guy is about to be in a lot of pain
Unfortunately, this is where you'll encounter Dishonored's conflicting nature: As fun and badass as the combat is (at times it really does make you feel like a master assassin), it doesn't really seem to want you to have that experience. The game makes a point of letting you know that the more people you kill, the darker the ending will be. Every mission you go on also has an optional non-lethal ending, which involves getting rid of your target without explicitly killing them. In addition, there's achievements for not killing anyone in the game, and on the mission summary, there's a small checkbox that takes note if you've managed to play the mission without drawing blood.
Having alternatives is a good thing, but it's odd that a game where you play as an assassin, and which has put so much effort into making killing really fun, tries so hard to sway you into the non-lethal options. There's nothing physically preventing you from going all out and killing everyone you see, granted, but if you're trying get the good ending, as me, it actually detracts from the game a bit. In particular there's a few assassinations which would have almost been artful in their execution, but which ended up being rather dull because I needed to keep my target alive.
You'll face a number of enemies in the game. The most common is the soldier, usually equipped with swords and pistols. Some times these will be accompanied by gun towers, and walking stilted soldiers called Tallboys, who fire incendiary shots. There's a few instances of thugs, as well as "weepers", zombie like citizens in the later stages of the rat plague. Finally, there's wildlife too. A few different variations exist, including a clam-like creature that spits acid, aggressive fish in the rivers and of course, the rats. Rats in large numbers will attack you fiercely, and if someone is caught in their frenzy, the swarm will literally devour them whole. I stared in morbid fascination the first time I saw them strip a corpse until nothing was left.
That's a tallboy. Creepy, even without the full moon and dead tree
Enemy AI is fairly standard; if you're within view for long enough, they spot you and all hell breaks loose. If one spots you, they'll call others as well. As I said, standard stuff. One might be mistaken for thinking the NPCs have horrible vision as you can usually lean out from behind a corner and they won't see you, even when you're very close; you aren't detectable unless you actually step out from behind cover. However, this is simply a feature of the game – it's designed that way so you can scope out a situation safely, and plan ahead. Similarly, whenever you approach a door, you'll usually be able to peek through the keyhole.
This is what dishonored is all about; it's about being an infiltrator. The challenge doesn't come from slaughtering everything in your path, even though you may indeed do so – the challenge comes from analyzing a situation, and then overcoming it. It's about thinking and executing. And every environment you're in is expertly designed to give you multiple ways to do this, and goes out of it's way to ensure that the experience is smooth and flowing. If you rush through, guns blazing, I fear you're missing the point. In many ways, dishonored plays like how I wished Assassin's creed would have played; it's not simply "approach target, then initiate scripted fight/chase scene" – it truly gives you the opportunity to apply your own tactics.