They hadn't seen each other that evening, but it could also be said they had never truly seen each other at all. They worked together on faith and hope; hoping the other person was someone they could trust and faith that other person really was. They were on a journey together, a journey which Walter Neff's co-worker and friend Barton Keyes had said ends only at the grave, but something stood between them; they had to depend on faith and hope because they didn't know each other well enough to have it another way. In the film "Double Indemnity", based on the James M. Cain novella of the same name, things were seldom as they seemed.
Walter returned home, confident they had accomplished the perfect murder. Phyllis called from the drug store and wanted to see him, and he agreed. But before she arrived, Keyes payed Walter a visit; something was on his mind about this murder case and he wanted to bounce it off a friend. If Keyes happened to see Walter and Phyllis together, it might help him find the answer. But for now, Keyes has no reason to suspect his friend Walter Neff has conspired with Phyllis Dietrichson to kill her husband and collect the insurance money.
Keyes keeps his visit short and is headed for the drug store to get something for the indigestion this case has given him. He doesn't know the remedy is waiting in the hall, listening through the door. If Phyllis hadn't heard Keyes, and had interrupted the conversation, it would have been over. As Keyes leaves Walter's apartment, Phyllis ducks behind the door. Walter has come out of the apartment to see Keyes to the elevator, but also to keep watch for Phyllis. She tugs slightly at the door, letting Walter know she's there, so he remains by the door.
Keyes starts down the hall, but turns from the elevator and returns to Walther, not realizing Phyllis is there too. The movement could be seen as symbolic of how Keyes is unknowingly moving closer to the truth. A few steps more would reveal the woman standing behind the door, just as a few more steps in Keyes' logic can reveal that same woman's secret.
As Janey Place and Lowell Peterson point out in "Some Visual Motifs of Film Noir", Barbara Stanwyck is lit in this scene with direct, undiffused lighting to give her Phyllis character a "hard-edged, mask-like surface beauty". This is the mask she always wears around Walter, and on the other side of the door, Walter appears soft and vulnerable.
The use of physical elements to convey psychological meaning was common in film noir. As such, the door in this scene gives insight to the entire film by representing the true nature of Walter and Phyllis' relationship. They stand in danger together, but at the same time they are separate. Right now it's just the door between them, but at other times it's the lies Phyllis has told Walter. Neither of them knows for certain who is on the other side of the door, but they've placed their entire trust in whoever it is. The door isn't all that separates them.