Thief takes place from a first-person perspective in a 3D environment, with the game's story taking place over a series of missions, in which the player character is able to perform various actions such as leaning, crouching, swimming, climbing, running and fighting, amongst other abilities. Levels are largely unscripted and maze-like, and allow for emergent gameplay; while non-player characters (NPCs) may either remain stationary or walk about on a patrol route, players have the freedom to choose how to get around them and the obstacles in a level's environments in order to complete specific tasks, such as getting through a locked door. In each level, the player is given a set of objectives to complete, such as stealing a specific object, which they must complete in order to progress to the next level; the player can choose to play on one of three difficulty settings before starting a level, which they can change between missions, with higher difficulties adding additional objectives such as not killing human NPCs or stealing a certain amount of loot from the amount available in a level, changing the amount of health the player character has, and changing how sensitive a NPC is to their environment. In some missions, players may find objectives being changed or new ones being added, due to certain circumstances they encounter, while failing a key objective or dying, will fail a level, forcing a player to either replay it or load up a previous save.
As the game's emphasis is on stealth, players are encouraged to focus on concealment, evasion, distraction, misdirection, and subtle takedowns, rather than on outright confrontation; the player's character can engage in sword-based combat when the need arises, and can perform three different attacks as well as parrying, but has limited proficiency and damage resistance in such circumstances. To do so, players must remain aware of their surroundings. To assist them in remaining hidden, a special meter on the heads-up display (HUD), in the form of a gem, helps to indicates the player's visibility to NPCs; the brighter it is, the more easily they can be visually detected, thus sticking to dark, shady spots where the gem dims, ensures the player is hidden, though NPCs can still find them if they get too close in front of them. To remain quiet, players must be careful of how much noise they produce, as well as what surfaces they are moving over; walking on soft surfaces like carpets and grass, is preferable as footsteps remain quiet, compared to walking over metal floors and ceramic tiles, which produce a lot of noise. NPCs also produce noise, either from whistling, or walking about for example, which can help players determine how far they are to their own position. Noise can be used by the player to mislead/distract NPCs, such as throwing an object to lure them elsewhere.
The game's NPCs feature artificial intelligence (AI) systems that detect unscripted visual and aural cues. If an NPC sees or hears something out of place, it will react to this, depending on the level of its suspicions; if for a brief second, they will simply ignore it, but if for long enough, they will become alert to their surroundings and begin searching the area. NPCs will react to things such as clashing swords or the reaction in other NPCs' voices, as well as to visual changes to their environment, such as blood stains, opened doors and fallen bodies; players can avoid leaving visual clues by cleaning them up, such as hiding bodies. NPCs are divided between three categories - "guards", "servants", and "non-human" - whose reactions vary; guards will call out an alert if they spot the player and attack them; servants will run for help if they spot the player or a body; non-human NPCs will merely pursue and attack the player. If a guard is significantly injured, he will try to escape and find help; some non-human NPCs will merely flee. Non-human characters range from giant spiders, feral creatures, to zombies and ghosts, with certain levels containing survival horror elements.
