The current Opera House auditorium is a Grade 1 listed building seating 2,268 people and has four tiers of boxes and balconies and an amphitheatre gallery. The stage has probably played host to some of the finest names in ballet and opera, whose names escape your author.
If you can't face sitting through hours and hours of Opera (or even worse ballet) you can still have a poke around on an official tour. Tours usually Monday to Friday at 10.30am, 12.30pm and 2.30pm, and on Saturdays at 10.30am, 11.30am, 12.30pm and 1.30pm, but this is understandably subject to changes and rehearsals to you should contact them to check. They take about an hour and a half and cost £10.
For more information, see http://www.roh.org.uk/whatson/tour.aspx