Many of the scenes are shot on the apartment building stairs, with the actors holding conversation whilst ascending or descending all 3 flights.
But actually floors 2, 3, and 4 are all the same set just re-dressed differently, this means that production has to halt why the 'new' floor is arranged, and then the script is resumed. What a pain!
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Things You Didn't Know About The Big Bang Theory
FanficHere are some mind-blowing facts that you probably didn't know about The Big Bang Theory.