The book that I was reading,"the Art of Voice Acting" said that to sound believable for your performance you have to have variety, tension, and sincerity.
What is variety? Variety is making a motion, like if you want to sound confused or surprised, then sound like your confused or surprised.
What is tension? Tension is like when your like wondering what is going to happen and like when it's all mysterious like "then Bob opened the suit case to find..." some people might wonder what is going to happen next.
What is sincerity? Sincerity is that if you have to do a grandpa voice, then actually talk and sound like a grandpa, and if you have to talk and sound like a little girl, then talk and sound like a little girl.
I think the easiest for me is variety, and the one I think hardest for me is tension.
I read all of this in the chapter