I found these on twitter:
*According to LE GAULOIS (which later serialised Leroux's novel), rumours soon spread that the entire chandelier had fallen.
*Gounod's FAUST was by far and away the most performed opera at the Palais Garnier when Leroux's novel was first published.
*Gaston Leroux originally intended Erik, the Phantom of the Opera, to have origins not in Normandy, but rather in Sweden.
*Leroux concluded the manuscript to PHANTOM OF THE OPERA by writing he was hungry, before firing a pistol from his balcony (Yep, we know that he did that, but it's really funny)
*The final version of LE FANTÔME DE L’OPÉRA omits a whole chapter drafted by Gaston Leroux, entitled 'L'Enveloppe magique'. (Haha, L'enveloppe magique means "The magic envelop" to those not familliar with the french language)
*Leroux had planned to write a story similar to the Phantom of the opera set in the Palais Bourbon, the seat of the French National Assembly.
*Leroux originally intended for the character of Christine Daaé in THE PHANTOM OF THE OPERA to be called Pauline Bellini. (And to make her italian)
*Monsieur Mifroid, the police commissioner investigating Christine Daaé's disappearance, appears in 3 other novels by Leroux. ( that's really sad. Knowing that another character wasn't real)
*The design of Leroux's torture chamber is based on the Palais des Mirages, created in 1900 & now at the Musée Grévin, Paris.
*Gaston Leroux 1st mentioned his Phantom of the opera novel in a letter dated April 1908.
*Gaston Leroux likely began researching the Paris Opera House in 1902, when he was sent there by his newspaper LE MATIN.
* In Leroux's novel, the Persian's description of the torture chamber owes much to Edgar Allan Poe's THE PIT AND THE PENDULUM
Here's an extremely rare picture of Leroux writing in his office: http://img.pikchur.com/pic_ITD_l.jpg?lm=