X: I am interested in the way people act towards each other.
O: I am interested in the way people behave towards each other.
When you are talking about what someone does on a particular occasion, act and behave are interchangeable. When you are talking about what someone does or what people do use behave. "Look how that boy is behaving."
X: He refused to accept responsibility for his act.
O: He refused to accept responsibility for his actions.
The noun act is usually used when you want to comment on a particular thing that someone has done. When you are talking about someone's general behavior use actions.
