Thief's Focus ability allows the game's protagonist to slow time and increase reaction times, but players can choose not implement to it.
That's according to an interview with Thief Game Director Nick Cantin, who said that the game can be completed without using the ability, should players want the extra challenge.
"Presently, it's completely the player's choice to use Focus or not," he said. "If players want to challenge themselves to stick to Garrett's basic abilities, they can."
Cantin also insisted the new mechanic isn't designed to make the game easier. "Garrett's Focus ability is there to empower the player when they need it, to give Garrett an edge over those surrounding him.
"But it comes at a cost," he continued. "It's all about giving choice to the player, but it will not break the game balance."
Elsewhere, it's confirmed that Focus will be a finite resource, and that it can be used to pick locks, pickpocket, and has several combat implementations.
Thief - the fourth game in the stealth series - was unveiled last month. It's coming to PC and next-gen consoles in 2014.