Are you listening, or not?

We had this event a while ago, and I hope it bears repeating:

https://www.youtube.com/watch?v=-BdbiZcNBXg

https://www.youtube.com/watch?v=aDMtx5ivKK0

I assume all of us have had the experience to not being "heard". We get the distracted, bored look, or someone waits for you to finish, and then they continue their monologue. People may even "talk pass" each other, where both are giving monologues, but don't even realize it.
Is this just the "listener's" fault, because what you're saying isn't interesting to them?
If it is of interest, do they want to hear every detail, or just a summary?
How does body language contribute to the feeling one is being listened to?
What phrases and words can help us listen, and give the perception we really are listening?