The short answer, I think, is that there is no "best" response when you feel your partner doing something unusual. Think of it as a conversation. She says something. While there may be a "best" statement that you can make in reply to what she just said, at that point you are reciting dialogue.
How are you?
Good. Thanks. And you?
Converstion? Not yet, I'd say.
I probably miss reacting to something out of the ordinary the first time I feel it. But, if it comes around again, I try to be ready for it.
This is the same thing that happens when you as a leader lead some next step that your partner is not used to taking. Often they will miss it. I think it's fair game to try it again at least one more time.
What I've learned to do while dancing, is to feel where our momentum would most easily take us if we just went with it.
I've done whole dances with people who felt like they were "inviting" me to step a certain direction, because there was less of a connection on one side than the other. After the dance I sometimes ask if they had that in mind or if it was just happening. This gives them a heads up that I am "listening" to them as we dance.
When you find the right teacher, (if you haven't already) they will be able to explain to you how you can do changes of direction, etc.
Just as Peaches wrote regarding being able to "speak up" as a follower, it takes time to develop the ability to listen and react as a leader. Make sure your partners who enjoy "playing" while dancing know that you enjoy it. You might also ask them if they were doing something on purpose, if they had something in articular in mind, who they learned it from, etc. (Short converstions between songs or during cortinas, off the floor, etc)