When choosing a game to create a let's play for, it's usually best to select a game you've played many times before, almost to the extent of knowing it inside out so that you can give the viewers a few tips and tricks along the way. The problem with having this knowledge when let's playing games that have actual storylines is that the narrator can't show his or her viewers their first reaction to the big plot twist in the game. Or can they?
Some let's players will feign ignorance to the plot in order to portray a sense of surprise when watching certain story elements unfold. While this is good if you don't want to spoil the game for your viewers, it can often come across sounding very fake. Especially if you've already revealed you've beaten the game five times or that you can't wait for the weapon you get after the next level. If you're not doing a blind run, it might be better to just keep your mouth shut during plot scenes instead.