When we were in Hollywood, we didn't go to Universal but booked the VIP tour at Warner Bros. The expectations were high. A real peak behind the scenes of a working Hollywood Studio was what the brochure as well as several travel guides promised. Security was rather tight. No Pictures, no chewing gums and no hopping off the golf cart even if we see a Star.
We did see the sound stage where they are producing ER -- from the outside -- not all to exciting. We also did see a sound stage from the inside, where they were producing a half hour sitcom with Michelle Pfeiffers sister in it.
Later they showed some of their open-air sets. This was quite interesting. It seems that on location shots are something you really want to avoid. So when ever possible they work on the Warner premises. They have several permanent sets there which are used in different films over and over again. There is the western town where everything from Little House in the Prairie to Bonanza and The Postman were filmed. Right next to it lies the back-street of a big city where Batman, Butchers Wife and ER shots were taken. In the canal right adjoin to the studio they shot scenes for The Terminator as well as Grease.
So if you get that deja-vue feeling again when you watch a new Warner Movie, this is why.