The reason why the Act could be the least important would be that it is the shortest Act.It could be discussed that the characters have already been introduced and that in through this act the audience is led to the end of the conflict and the resolution of it all.There is a moment , a climatic, one in which Titania agrees to give Oberon the Indian boy but it is described and not seen on stage " ...which straight she gave me.."....and now I have the boy..."
