Singing and speaking come from the same instrument and the purpose is to COMMUNICATE A MESSAGE. The singer who is not used to moving arms or head while communicating is not being a serious communicator. The WORDS need to be conveyed from the entire body/being of the singer. Before a song is sung, ask yourself "What do I want the listener to do or feel from the words of the songs?" Next, set the ATtitude - "at oneness with the message" with your hand/arm/shrug of the shoulder/body posture. If it is a song with many repetitions of the same melody, there must be a body movement. So many times the novice singer does not pick up this concept because when singers are filmed on TV, the cameras are doing the movements and blocking the viewer from seeing what the singer's body movements are doing. This is NOT ACTING in the sense you are insincere. This is BEING THE SONG.