Winifred Watson's book was published the fall of 1938 and it became a smash hit. Plans were in the works for a Hollywood film version starring Billie Burke as Miss Pettigrew, but the start of WWII brought those plans to a halt. The publisher re-released the book in 2000 which lead to it being "found" again by Hollywood after 60+ years.
On the balcony, Miss Pettigrew is talking to Edith. The two soldier extras behind Edith are in the shot. Edith switches places and the same two extras are in the shot on the other side of the balcony. As Edith switches back to the first side, the same two extras enter the balcony from the sides she is on. There's no way for them to have gotten from one side of the balcony to other in half a second.