Harold Hill is a great role for a young person to play. Select a boy with charisma and charm, who is comfortable on stage. He should be a great actor, an average singer, and an average mover. You'll also want to cast a boy with a changed voice. For your sanity, make sure you cast someone who memorizes lines easily and has a good sense of musical rhythm. Your Harold should look good with your Marian and the two together should exude a spark of excitement.