Behind That Curtain is based on the third Charlie Chan novel written by Earl Derr Biggers. The book was fun pulp with Charlie outsmarting three rival detectives in solving the murder of a Scotland Yard detective and in turn solving two 15 year old mysteries.
The film, however, is interested in the two 15 year old mysteries and re-works the plot, so the film ends up being about Biggers' back story rather than the Chan story. Fox must not have been confident in the Chan character, perhaps because this was the first real year of sound film or they felt the audience would be more interested in the lovers and not a Chinese detective. Indeed, there is little romance in the book and the film takes liberties in changing 3 non-romantic characters in the book into a love triangle in the film.
It is rather sloppily done and the film really would be of no interest at all today, if it did not have small appearances by Boris Karloff and the Charlie Chan character. It is unfortunately a typical early sound effort and is cinematically uninteresting. Therefore the plot, which is not a mystery as the killer is revealed in the beginning, is all about the overdrawn lovers. It was probably even boring in 1929, but it qualifies as a curio today and should be viewed as such. It is interesting to see E.L. Park play Chan, albeit for five minutes and Karloff looks as menacing as ever. What cruel irony though, that no one can find "The Chinese Parrot", by the brilliant Paul Leni, nor the 4 missing Warner Oland Chans, and this is the one missing Chan that was found!!! Fate works in strange ways.