A feeling of having no control over a situation has been identified as one of the main sources of stress...which is a good reason to find ways to put your character into situations in which he or she has no control
As unpleasant as it is in real life (it's also been identified as a trigger for depression), it's an excellent way to increase the stakes for your protagonist. Of course, in real life sometimes people just curl up into a ball when under extreme stress, but that wouldn't make for a very exciting screenplay or novel.
Your protagonists will have to fight to regain control, and the less control they have the harder they will have to fight. The middle part of most novels and screenplays is a back-and-forth battle, with alternating gains and losses.
I've posted before about a useful structure for this--Maslow's Hierarchy of Needs, which often is represented by a pyramid with the base being the most basic needs (food, clothing, shelter) and the top being the most abstract and elusive need (self-realization). You can take your poor protagonists down the levels until they're fighting for their very survival.
I found this useful recently when trying to figure out how to add pressure on my protagonist in the middle of the story, so give it a try if you find the middle of your story sagging, too.
