Originally, the protagonist in Die Hard says his name is "Roy" in an effort to conceal his identity.
Somewhere down the line, his ally Al Powell and everyone else outside know he is John McClane. How?
And later on the boss even figures out that John's wife is among the hostages, while not noticing it previously. How does he suddenly know that?
Based on comment and based on my first view of the movie, it has something to do with Ellis talking to the bad guy about John. It wasn't very clear though.
Also if Ellis talks to the boss, then how come the cops know it too? So a related question will be, how are John, the terrorists, and the cops in the same frequency?
Basically, how do Hans, and Powell, start realizing that Roy is John McLane?