What is the difference between say an internal act and an external act?
I would venture to say that the internal is an “imperfect” act, and the external is the “perfect” act (the act in its completion).
An internal act is in potential to the external, and the will must move the body to make the real world match the “imagined reality”.
That is what “entering into temptation” is about - when a temptation first appears in your thoughts, your first act is to visualize what it would be like if you carried out the deed, imagining the pleasure that might happen. and when you begin moving to make it real materially - that is entering in, where the imagined reality takes material shape in the world.
Watch and pray, so you do not enter into temptation then would mean, be aware and seek grace to recognize the inner beginnings of imagining yourself participating in sin, so that you do not get up and actually start doing it in external act, but instead cease the internal act.
An internal act would be one that is not perceived. Only you know about it. An act of the will is an internal act. Mental prayer is an internal act. Only you and God would know about an internal act.
An external act is one that can be observed, For example if you run from here to there. The act of running is an external act.