Originally Posted by catachrest

This might be a regional difference - ie, maybe different dialects use it differently. In North America one can definitely use "to act out" to mean "to misbehave", and it usually has a connotation of acting badly out of a desire for attention. We also use to "act up" but a person probably wouldn't act up. A machine or other object (eg. a body part) might act up.

eg. My nephew's been acting out a lot lately. I wonder if everything's all right at home.
eg. The engine in my car is acting up. I'd better bring it in to the mechanic.
eg. My bad knee has been acting up again, and it's really sore.