In the movie 10 Cloverfield Lane, at some point Michelle and Emmett decided to escape the bunker. Why didn't they just disarm Howard and tie him up before anything else, so that they would have no fear of their plan to be revealed?
Because how could they? Howard is a large, paranoid man, who had a bunker built, sleeps in a locked room, goes around armed all the time, is prone to violent outbursts and dangerous actions.
Howard probably has 150 pounds on Emmett, who has a broken arm, and 200 pounds on Michelle, who's physically and mentally dwarfed by Howard. And that's being conservative, as John Goodman has been estimated at losing 100 pounds from over 400, between the filming and the opening. Estimates, Howard at 325, Emmett at 175, Michelle at 125.
Essentially, they had no way to disarm, tie him up, or subdue him. Emmett is injured, Michelle still hasn't gone through her trial-by-fire transformation (her character arc takes her from running away from her problems, to intentionally heading to the heart of an alien invasion), and Howard is strong and dangerous. Subterfuge is the only real option they have.