When you overhear, you hear, especially by accident, a conversation in which you are not involved. ex. I overheard a conversation between two boys on the bus.
When you eavesdrop, you listen secretly to what other people are saying. ex. We caught him eavesdropping outside the window.
When someone is a fly on the wall, he/she watches others without being noticed. ex. I’d like to be a fly on the wall when he tells her the news. In British English, when somebody’s ears are flapping, a person is trying to listen to sb’s else’s conversation.
When someone is all ears, he/she is waiting with interest to hear what someone has to say. ex. ‘Do you know what he said?’ ‘Go on, I’m all ears’.
When somebody’s ears are burning, a person thinks that other people are talking about them, especially in an unkind way. ex. Stop gossiping so much about her. Her ears must be burning!