What you should do depends on the situation you are in when you see your idol. If it is a mass gathering, then movie stars expect people to be asking for autographs on paper or clothing. If the movie star is in a private setting, not on a publicity appearance, then s/he may want not want to be bothered. If it is something you just can't refuse doing, then be polite and say,"Excuse me. I am sorry if I am bothersome, but it would mean so much to me if I could get your autograph."