behind closed doors 舞台裏では

behind closed doors は秘密裏に、裏では、こっそりと、、、、文脈に沿った解釈をすればいいでしょう。注意する点は behind closed doors と冠詞がつかないことです。0:18 then they met behind the closed doors, words of advice and the codes of France's nuclear deterrent were exchanged. オランド大統領は奥の間でマクロン新大統領に大統領職の心構えを伝授し、核兵器の鍵を手渡した。

Emmanuel Macron takes over as French president

https://www.youtube.com/watch?v=4NzpZ4rgvZE  0:22 but behind closed doors, former Obama Administration officials tell CNN, President Obama offered then President-elect Trump a clear warning. don't hire retired Gen. Michael Flynn as national security adviser. 執務室でのオバマとトランプの初の顔合わせは和やかムードだったが、オバマはトランプに別室でフリン氏を大統領補佐官（国家安全保障担当）に起用しないように忠告していた。
Source: Obama warned Trump not to hire Flynn

https://www.youtube.com/watch?v=pHwJHDQHbjA  2:25 a lot of times, abuse takes place behind closed doors. 虐待行為は往々にして人前では見せないものだ。
Mom who won custody of 2 kids featured in YouTube stars' prank videos speaks out

https://www.youtube.com/watch?v=w1TCoU948lQ  0:01 here in Silicon Valley we're a stone's throw away from major innovators. but behind closed doors, folks from one of the world's oldest professions, they're taking notes from their high-tech neighbors. 裏の世界では
High-tech sex workers in Silicon Valley

練習問題解答
a-5271 behind closed doors
b-4885 behind closed doors