I'm not sure if writers developed the entire book in their head first, but: it was indeed very, very common for people to dictate novels, journal entries, and other "written" works to a secretary, typist, or tape recorder.

Nowadays that seems to be rare, but my impression from reading my journals is that it was often more common to dictate than to physically hand write things.

Novels were dictated, that is absolutely correct, but on top of it, the whole plot was developed with a high level of detail before dictation started. There was some editing, of course, but nothing like we do today, where writing books is basically an iterative process. We lean on the written word too much for our thinking (not being critical, just that’s how we are taught)