Put simply, if there’s a dead body on page 5, the entire rest of the book is the discovery of how it got there in the first place. That means that dead body usually has a hefty backstory.
So here we have a diagram. The story begins at some point in the past, when the events leading up to the Inciting Event happen. The novel begins on page 1, and the dead body, which is the Inciting Event, appears shortly thereafter. In mainstream historical fiction, the possibilities for the inciting event are broader than just a dead body: the Titanic sails, a queen is crowned, a spy is put under house arrest.
What’s helpful to borrow from mystery, though, is the awareness that the Inciting Event operates rather like a fulcrum. If you want the story after the Inciting Event to be hefty and juicy, you need to have a hefty, juicy backstory.
Now for me, backstory is all about character. Backstory comes from what your characters experience in two realms: the Worldly Realm and the Personal Realm.
This was my first book, and because it came out of the research I did for my dissertation on railway disasters, the worldly realm was relatively easy for me to produce. In this novel, the daughter of an earl and her mother are in a railway disaster at the end of chapter 1. This is the inciting event, with lots of dead bodies. Elizabeth gets herself and her mother out of the burning railway train into a field, where they’re helped by the railway surgeon Paul Wilcox, and together with a newspaperman named Tom Flynn they uncover a railway sabotage and stock fraud scheme that traces back to the early days of railways, when Elizabeth’s father was a member of a corrupt railway board.
The other thing I do is write up 5-8 specific personal experiences that will shape my main character’s secrets and desires and cause her to forge some problematic assumptions about herself, her family, and the world. I write this from the "I" (first-person) perspective, and usually it's around 3-4 pages at first; it expands as I write the novel.
The baggage your MC brings to page 1 begins with backstory. Baggage is a good beginning.