Do you know what is あいづち(Aizuchi)?
Aizuchi is a part of Japanese culture. It refers to non-verbal responses you make when someone is speaking to you, to show that you are listening to him/her.
It is polite to show that you are listening and following the conversation.
Aizuchi is also an important part of Japanese business etiquette.

There are things you should NOT do when responding to the speaker, so please take note!

1. Leaving your mouth open (or half-open)
It shows that you are not serious about listening to the conversation.

2. Lifting your chin(あご) upwards
This gives the impression that you are making fun of the speaker.

3. Smirking (ニヤニヤする)
This conveys the message that you don't actually agree with the speaker and is only pretending to listen.

4. Saying 「うんうんうん（meaning "yeah yeah"）」or「はいはいはい　（meaning "yes yes" or "I know I know"）」in repetition
Conveys to the speaker that you are actually not interested in listening, but have no choice.

Besides the above 4 points, basic listening manners such as not interrupting the speaker should be observed.