Approximately 8 minutes into the film, when the cops show up at Marian's apartment, one of them asks, "What's happened?" Marian replies simply, "There's been a shooting. I did it." But her voice is obviously dubbed, and it's clear from the way her lips move that she said something else entirely!
Any lip readers out there who can tell us what she actually said?
She said "Get me out off this shà tty movie"!
You`re my wife now.
SPOILER:
I'm not a lip reader but I suspect that Marian originally didn't admit, at least for the first time, to the killing at that precise point in the film.
As it stands, this is the first point in the film that she has claimed, in front of the audience, that she was the killer (up to then there was room for ambiguity) but there is absolutely no emphasis on this at all. She is in profile in a wide master shot. In a studio film of the time, and even now, this would be highly unlikely to be the sort of set-up intended for such an important revelation as the female star of the film claiming to be a murderer.
There is pretty much zero reaction from any of the policemen, as if they expected this before they arrived.
Going back to the phone call to the police, we fade out before she is put through.
Possibly there wasn't planned to be a fade out and she originally admitted to the shooting in the phone call.
Having cut that scene they needed to clarify for the audience that Mariam was admitting the crime as soon as possible and so used the opportunity of that wide shot to dub in the admission. Perhaps she originally said, in the wide shot, something like "I'm Miss Washburn, I'm the one who called the police", which would be all the police would need to know if she had already given them details on the phone. (Of course there could have been more dialogue which the editors discarded when they cut to the following closer shot.)
If that is the case, why they cut the phone call at all remains a mystery, although it could be as simple as wanting to pick up the pace of the action.
Alternatively, it is possible that in a flurry of rewrites and the semi-chaos of production that no one realized that Mariam hadn't unequivocally said that she had shot Susan before the first policeman tells the inspector that she has said she "shot a woman", and so her admission had to be slotted in earlier.
And lastly, it is possible that the conversation when the police first arrived was originally longer and only by dubbing in the admission immediately in the long shot could it be shortened.