The Thirty-Nine Steps shows...an attractive man, not too young...and not too old, since he must have the knowledge of maturity and substantial experience on which he will draw while being able to respond to the physical rigors of chase and pursuit. Let the hero, who appears at first to be relatively ordinary, and who thinks of himself as commonplace, be drawn against his best judgment into a mystery he only vaguely comprehends, so that he and the reader may share the growing tension together. Set him a task to perform...Place obstacles in his path the enemy, best left as ill-defined as possible, so that our hero cannot be certain who he might trust. See to it that he cannot turn to established authority to help, indeed that the police, the military, the establishment will be actively working against him.

Then set a clock ticking...