Here's why:
* There's no record of such a conversation: No one close to either actor, or involved with the production of "The Dark Knight," has ever corroborated this story.
* Nicholson's performance was highly respected: Nicholson's Joker in Tim Burton's "Batman" was considered a classic, and it's unlikely he would have felt threatened by another actor taking on the role.
* The roles are very different: Nicholson's Joker was more cartoonish and comedic, while Ledger's was a much darker and more psychologically complex character.
It's important to remember that stories like this often spread online without verification. It's always a good idea to be critical of information you find online and to check for reliable sources before accepting it as true.