I read "Double Indemnity" and was amazed to see to see that Cain gave his brilliantly-conceived characters the short shrift and spent most of his time on trivial details of the crime. I also read the novel of "Mildred Pierce", and that had the same problem. Once again the characters and situation were brilliant, but Cain didn't know what to do with them. That's OK. He was still a brilliant writer.
He had six famous writers (one of the was Faulkner) take a shot at it before he got what he wanted. It was frustrating because combining two separate genres isn't easy and he didn't know how to go about it. He just had a feeling that he'd know it when he saw it.
Sometimes he had three writers working simultaneously and completely separately on the same project, a practice that makes writers furious. One writer "broke the spine" of the improved story, but was too slavish to the book in the details. Two others added too many fantastic and implausible elements, and made the story too long. One made the story too violent. Wald believed melodrama couldn't support too much violence. One murder was enough.
Finally he had enough interesting scenes to make a good story. Every writer contributed something of value, but it was still too long. With the shooting date approaching he took the bold step of getting a radio writer to condense the story. Radio people were experts at telling long stories in short formats. The radio guy, Ranald MacDougal, accomplished miracles and tied it all together deftly.
At one point in the story Mildred marries a guy and one minute later -- one minute! -- she decides to divorce him...and it works! Now that's compression! MacDougal did it by making the audience hate the guy and want to see Mildred divorce him. We're actually impatient to see Mildred dump him and when she does it, after only a minute of screentime, our only reaction is "Well, it's about time!" MacDougal actually makes the guy appealing in certain other parts of the film, he just emphasized the negatives in this section to smoothe over the story compression. Wow! Is that expert writing or what!?
I forgot to add that Wald hired Curtiz to direct the film, which was a brilliant choice. Curtiz injected humor into the story to smooth over the sometimes fuzzy logic, and it worked beautifully.
I'm not aware that Jerry Wald did anything else that was particularly distinguished, but in 1944-5, when Mildred was made, he was definitely cooking with gas. The womans' film/noir synthesis he created is now one of the most common types of film.