You are right in that so much of the work in bringing a screenplay to the screen - small or large - comes after the script is complete. Research is still critical, however, just the focus of that research is perhaps less detailed.

Take the Edwardian period, for example. For the screenplay I would not likely have to make detailed research into clothing and fashion, unless an item of clothing, or a character's interaction with it, was an important element of the story. Let's say the mystery is solved by a fingerprint on a watch. It would be vital to research what kind of watch that might be based on the character who owned it and the situations in which it might be used, and critical as to whether fingerprinting was used as a means of solving crimes at the time - it was, by the way, the first person to be convicted as a result of fingerprint evidence was in 1902 when Harry Jackson was convicted of breaking and entering, and the first fingerprint conviction for murder was in 1905.

TableRappers is an interesting case in the research. The initial incarnations of the story were wholly period-independent as they surrounded the personalities, characters and relationships. Once the Edwardian period was chosen, I was able to then give those characters context, dialogue style, attitudes, Edwardian ideals and motivations etc.

This was one important aspect of selecting that period in that I wanted the freedom to write openly strong female characters, something that would be much more plausible and accepted by with the Edwardians, yet, as I am writing in the early Edwardian period, still some conflict as the remnants of the much stricter Victorian period were still a major influence. Thus the relationship between Keynes and Lady Katherine becomes an interesting balance between her outward independence, and his residual traditional ideas conflicting with his close friendship with her. This relationship, I suppose, reflects certain aspects of that age as a whole - the transition that society was living through at that time.

I'll get back to answering your question now that I have waffled a bit ;)

There is much more detailed research for the books, but primarily because there is simply much more information in the books, more words, more descriptions, and more detail about places, people, rooms, objects, etc.

For me, however, it is important that I do not get too wrapped up in the finer details of the Edwardian period to a point that the story itself suffers. These books are not historical refernces. I would much rather face the wrath of the Edwardian expert over some minor detail of the time than spoil your enjoyment of the story.