Every action causes consequences, reactions that create contradictions or challenges to that action. These challenges and contradictions create a crisis in the story line. A crisis situation is a moment when something must change because the action cannot proceed as it had before this moment, because the contradictions of the action work against the goal and the motivation for the action.
For instance, if a character steals a car, this act will have consequences. Someone will start looking for this car. The crisis will take place when the car owner, or the police, find the character who stole the car. This is a decisive moment in the story, it is a moment of confrontation, and the resolution of this moment will have profound effects. In this moment of crisis, the thief must make a choice, which will reflect his character nature. (Of course, the act of stealing the car was, too, a choice, and too, a response to a moment of crisis, lets say, the need to get away) What he cannot do is to proceed as he had before--to keep driving the car with impunity.