If you are planning to include a character following a particular profession, having a background in that profession can be very useful. Take for example, Bernard Knight the former forensic pathologist who has written two very successful historical mystery series; the Crowner John novels set in medieval Devon and the Dr Pryor forensic mystery books set in the 1950’s. Obviously you can’t have a detailed knowledge about all careers – so the most valuable thing here is knowing someone you can ask: “Who do I know?” Generally speaking people are generous with their time and are quite happy to share their knowledge and expertise.
If you have decided to concentrate on a specific historical period in your writing, go back and browse your personal library, fiction and non-fiction. It is likely that you are drawn to a particular period for a reason – no doubt you will be adding to that core collection. Elizabeth Chadwick has generously shared her reference library for the medieval period on her website – the titles marked in green are those which she’s found particularly useful.
If you are going to set your story in a particular location, then get to know the place well. You may live there, or visit regularly. Buying a house in Carcasssonne lead to Kate Mosse setting her Labryrinth trilogy in the Languedoc, France. In an interview she admitted that she had thought that the first book would have been set in ancient Egypt. (1)
Tracy Chevalier mentioned her frustration when walking around Lambeth as background for her novel Burning Bright, because there is little left of the streets and buildings that William Blake would have known. The problem was resolved by her relying instead on eighteenth century paintings, drawings, and maps. (2) To gain insight into the period detail of a place, visiting the local museum, or heritage centre can provide a wealth of historical information.
By asking the question “What do I already know about X ?” you may well find that your personal knowledge is greater than you first thought – if not, then you know the areas with which to begin your historical research.