Secondary action is similar to overlapping action, and it can be difficult to determine if an action is secondary or overlapping at times.
The difference is that overlapping action is the result of the main action, and secondary action is independent of the main action.
For example, an alien with antenna might be walking (the main action), while the alien’s antenna is moving around to sense the environment (secondary action). Whether the alien is walking or standing still, the antenna continues to sense the environment. Its movement is independent of the walking.
Secondary action should remain minor when compared to the main action or their roles could be reversed. It should support the main action and make it more interesting.