How we change what others think, feel, believe and do
'Acting out' means literally means acting out the desires that are forbidden by the Super ego and yet desired by the Id. We thus cope with the pressure to do what we believe is wrong by giving in to the desire.
A person who is acting out desires may do it in spite of their conscience or may do it with relatively little thought. Thus the act may be being deliberately bad or may be thoughtless wrongdoing.
Where the person knows that they are doing wrong, they may seek to protect themselves from society's eyes by hiding their action. They may also later fall into using other coping mechanisms such as Denial to protect themselves from feelings of shame.
An addict gives in to their desire for alcohol or drugs. A person who dislikes another person seeks to cause actual harm to them.
Acting out may be considered as actually not coping, although it is handling the pressure by giving in to one side, whereas most other coping mechanisms seek to handle the pressure of not giving in.
A person who is acting out may decide to 'repent at leisure', seeking the pleasure of the now by mortgaging future contentment. This may be caused by cognitive short-sightedness or by contrarian tendencies.
Acting out is an opposite of sublimation, whereby a desired behavior is displaced into an acceptable activity.
Help people who are acting out by highlighting how ashamed they will be later, such that when they consider acting out in future, the later shame is significant enough to prevent their acting out now.
If the behavior you want is outside the other person's values, you can encourage them to act out, for example by promising to keep their behavior secret. However, this will not stop them (and you) from using other defense mechanisms later to suppress feelings of shame.