What's with the "one-sided" dialogue scenes in Midnight in Paris?

I re-watched Woody Allen's Midnight in Paris last night. I noticed that during a lot of dialogue scenes with only two or three people, there will often only be one of the characters in the shot. It gave me the impression that those characters' lines were shot separately and then edited back into the scene, but I'm not sure that that's actually the reason why they are shot this way.

An example off the top of my head is the scene in Gertrude Stein's salon when we first meet Adriana, the Marion Cotillard character. She is standing in the doorway of the salon while everyone critiques Picasso's portrait of her. During her lines, she is the only person in the frame, and if I recall correctly she doesn't have any off camera lines, either.

You can almost see it in this video but it stops too soon. (I will keep looking for a more demonstrative clip.) The rest of Adriana's shots in that scene are framed exactly like that first shot of her, close in on her, with no other people in the shot and no movement from the camera. It gives me the impression that she isn't really a part of the rest of the scene, like we're cutting to somewhere else when we're looking at her.

Is this a choice, or were there simply scenes that had to be re-shot later and so they ended up a little funky? I'm not a big Woody Allen buff - is this something that he does frequently? If so, what is the purpose of shooting specific bits of dialogue like this?

Update:
I found another clip that sort of demonstrates this. It's when Gil first meets Salvador Dalí in the restaurant. I'm not sure this is the same cut that's in the film - I don't remember that long of a shot on Dalí when we first meet him and sit down at the table. But regardless of that issue, once we get into the scene, Gil's two lines ("Rhinoceros?" and "complex situation") are shot in the same isolated way as Adriana's first scene.