Casting is a tricky business. Many times, writers and/or directors have a particular actor in mind from the conception of the project. If they end up getting that star, it's great. If, for some reason, they don't, there can be a scramble to find someone else who fits the criteria and is available. Cinema history is littered with times filmmakers didn't get their first choice, only to see the movie falter.
And then there are the happy cases where an actor played a role that was written for someone else and nailed it. Luck isn't the reason why - talent is. Even if the writers and directors envisioned a different person at first, these actors were clearly as good a choice, if not an even better one. In the best of examples, it's now hard to imagine anyone else playing those characters. That's how fully they embodied the parts. The following stories of how one performer effectively replaced another will increase your appreciation of these popular movies.