Instruction giving can be used in several applications, ranging from trainers in simulated worlds to non player characters for virtual games. In this paper we present a novel algorithm for rapidly prototyping virtual instruction-giving agents from human-human corpora without manual annotation. Automatically prototyping full-fledged dialogue systems from corpora is far from being a reality nowadays. Our approach is restricted in that only the virtual instructor can perform speech acts while the user responses are limited to physical actions in the virtual worlds. We have defined an algorithm that, given a task-based corpus situated in a virtual world, which contains human instructor's speech acts and the user's responses as physical actions, generates a virtual instructor that robustly helps a user achieve a given task in the virtual world. We explain how this algorithm can be used for generating a virtual instructor for a game-like, task-oriented virtual world. We evaluate the virtual instructor with human users using task-oriented as well as user satisfaction metrics. We compare our results with both human and rule-based virtual instructors hand-coded for the same task.