What happened was that Carlino slipped a piece of plastic over the bolt, or rigged it some other way-- he might have even removed it, or removed the spring; it was an old bolt, the kind that was put on interior doors that originally had no door knob key, when brownstones were turned into apartments in New York. Now all those doors would have deadbolts, at the very least, and maybe even metal doors. But they didn't in 1967. Only the exterior door had a deadbolt.
The lock on the apartment door isn't a deadbolt (which is by definition, a bolt without a spring). The bolt has a spring, and will lock when the door closes, if a button has been pushed, or slid to one side. If it is set to lock, you have to turn a knob to open it from the inside, or use a key from the outside. With this kind of lock, you don't need to re-lock it every time the door closes; if you leave the button set, it stays in “lock” mode. You would need to let someone in, when it's locked, but not lock the door behind them, and you would need a key to get in, but the door would lock again when you shut it. This is probably a reason the locks aren't seen much anymore. It was easy to forget you had it set on "lock," and lock yourself out. So, if Susy thinks it's set on “lock,” she doesn't think she needs to go to the door and lock it behind everyone.
She did, in fact, lock it, but unfortunately off-screen (probably when she and Mike were looking for the doll), she thinks it's done, and she doesn't need to follow everyone to the door. But I think "follow everyone to the door" is what Ebert means when he says "Why doesn't she lock the door?" Since you don’t see this type of lock much now, I understand current audiences thinking Susy is being dense, but Roger Ebert wrote the "Susy is stupid" review right after the film came out. Maybe it Ebert himself had never seen this kind of lock. He may have never lived in a subdivided building, and somehow, never managed to pay attention to how the lock worked when he visited friends, but it seems to me that he was the one being dense. Yes, the “lock” sequence is sort of hard to follow, but he is a professional reviewer.
Early in the film, the lock is set to open, because it is open when Mike visits Susy the first time, and when Gloria comes for the grocery money-- it's probably open for Gloria, although she has a key.
It's locked later, after Roat, sr. has come in, and probably as a result of that. We know this, because Gloria needs to use her key, and Carlino finds it locked when he comes the second time, by himself. He has to wait for Susy to let him in. It's AFTER this incident, when he realizes that they do sometimes lock the door, that Carlino rigs it.
I think the problem everything isn't clear is the editor's, not the writer's, and not Audrey Hepburn's.
The first time after she has set the bolt to "lock" that Mike saunters in, Hepburn looks surprised, because she knows she locked it, but it's the same time she's trying to cover up the fact that she has been trying to call the police, and not let Mike know she has figured out he is involved in Roat's plot. It's a lot to ask for her to add an extra layer of shock that the door wasn't locked, as it should have been, so even though the camera has caught Carlino rigging the bolt, I think a lot of audience members don't quite catch what just happened. The problem is that we never actually see Susy deliberately setting the bolt. We just know she did, because Gloria needs a key, and Carlino has to be let it, so she must have locked it. We need an actual shot, a close-up, of Susy locking the door, and a little gloat, or something, from Richard Crenna, when he comes in. And possibly something passing between Mike and Carlino, communicating that the door has been "done." I don't remember whether there's an opportunity for that or not, though.
We do see Susy check the lock once, when she runs upstairs with the doll. It's the same moment she realizes that Carlino is also in on the plot to get the doll. It's possible to misinterpret is as Susy about to run outside, then changing her mind, when she realizes the police can't help her (she isn't wearing a coat in the winter, but that's a lot to ask an audience to think through).
At any rate, Ebert is the guru of film opinions, so now "Susy is stupid," and the question of the lock is out there, and reeling it in is pretty much impossible.
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