In the lexicon of acting tools, the word 'actions' can have different meanings. For some practitioners, the word is interchangeable with 'objectives.' In some contexts, 'actions' refers to physical activities. In 'The Complete Stanislavsky Toolkit,' Bella Merlin writes about how Stanislavsky considered actions to be both internal and external. An internal—or psychological—action leads to a physical action.
Here we'll confine the discussion of actions to the psychological, not the physical. Thus, actions are the choices you make, beat by beat, to affect the other characters in a way that will lead you to achieve your overall scenic objective. Actions are expressed as active, specific, dynamic verbs. Those verbs must represent something you can actually do, a way that you can influence the other characters. You can't change what the other characters say, but presumably you can change how they say it, how they feel and behave. If one action doesn't work, on to the next. If it does have the desired effect, you'll still need to find another in order to build toward achieving your ultimate objective, because there will always be obstacles.
Strong objective, strong obstacles, strong actions—that's what it's all about. Los Angeles acting teacher Ivana Chubbuck defines actions succinctly in her book 'The Power of the Actor': 'Actions are mini-objectives attached to each beat.' She explains that actions allow you to go after your objectives with specificity and a range of behaviors.