Confusing movie

I don't mean that to say the plot is too difficult to figure out, far from it; the twist is well explained.

It is confusing because I have no clue how to judge this film. First, the cockney woman who has the letters is obviously Marlene Dietrich with a wig and a bad accent. It is so patently bad it reminded me of the horrible prosthetics that Yaphet Kotto wore in the James Bond movie. But are we supposed to tell it was her? Was Billy Wilder trying to trick us with a fake twist only to trip us up with the real one?

Second, the acting... Both Tyrone Power and Marlene Dietrich are decent actors but their performances in the courtroom are so bad it made me wonder if they were supposed to look hammy (as they are supposed to be faking their performances within the court trial).