The single most crucial aspect of a movie is its cast. Without having the right actors, even productions with the finest directors, enormous budgets, amazing scripts, and high-tech special effects will fail spectacularly. Unfortunately, all it takes is a single miscast to bring a good movie down.
Of course, some movies are terrible – they get everything wrong, including the whole cast – but that's not what we'll be talking about here. Instead, we're talking about one casting choice that ruined what would otherwise have been an at least semi-enjoyable movie.