Billy Zane pumped iron for over a year to fill The Phantom's costume. A Batman-like costume with fake muscles was reportedly made, but by the time filming started, Zane was so beefed up that he did not need it.
Billy Zane is a huge fan of the Phantom, having been introduced to the comics while filming Terror a Bordo (1989). Zane has also called this his personal favorite film he's made.
One of The Phantom's trademarks in the comic strip, his striped "underpants", was tried on the movie costume, but reportedly looked too silly to use in the final film.