keep (one) guessing
To withhold information from someone so they don't know what will happen or what one will do next. I feel like my girlfriend is always trying to keep me guessing. I just never feel totally comfortable with her. In show business, it's important to keep your audience guessing, to keep them engaged.
keep somebody ˈguessing
(informal) not tell somebody about your plans or what is going to happen next: It’s the kind of book that keeps you guessing right to the end. OPPOSITE: give the game away