This may be something I remember from physics or it may not be since that is the only college course I failed to ace. I did not fail but nearly. For every action there's an equal and opposite reaction. For characters part of this may not be true though I;m not sure.
Just what is an action in fictional works. The action can be physical or verbal, usually. If someone slugs the hero or heroine with fists or with words, the hero or heroine is going to react. This reaction can tell you much about the character.
Since much of the way I develop my characters is by using a bit of Astrology, their reactions are usually geared to their signs. An Aries would hit back and usually not with words. A Scorpio wouldn't get mad but they would get even. These are just examples of reactions.
The kind of action the character chooses will also delineate their character. Are they upfront, subtle or some variation of these two. An action can be deliberate or unintentional. We all know people who open their mouth only to shove their foot inside. There are some people who will never act openly.
An action or reaction can come from outside the character or be an internal one. Think of how people operate during an emergency. Some panic and some take charge. Or when a value is challenged. Some may act in the open and some may do so in secret.
When developing your characters, imagine how they would react in a given situation and when putting them into action, think of how this particular character would act or react.