About the Research
So, I'm guessing you were probably listening to an episode of the Murder Mile UK True-Crime Podcast and thought "how does he know all of those details about those murder cases, some of which happened years, decades, if not more than 100 years ago?"
Answer? Research. Lots and lots of research.
Every murder case has its own challenges, as court documents only give you a very cold appreciation of the case, they're very factual, clinical, dry and often miss-out key facts about who a person is, how they look, what they were thinking and what their life was like in and around the time of the murder, which I feel is vital to understanding a murder case. What was the victim / murderer's mood like before the incident? What was their mindset, their routines, fears, worries and lifestyle? It's these small details that are important.
To cover the factual elements of the case, we're lucky because many British courts (whether magistrates, crown or coroner's) are slowly digitising many of their cases for research purposes, even though many cases have a 50-75 year moratorium on them, so the only way to get eye-witness testimony is by interrogating numerous elements in newspaper articles from the era. Obviously given the British media's dubious track-record in "misinterpreting the facts" to suit thier own story/agenda, I use multiple sources to corroborate what they report. I also spend many hours, days and weeks in the National Archives going through the original case files, which is an amazing experience as sometimes you get to touch and flick through a detective's actual notebook, read his personal thoughts on the case, and - which has happened more times than I care to mention - I've discovered a new fact, which was left out of the trial (for whatever reason) and it has given me a clearer insight into the murder.
That said, you can't beat a first-hand account from an eye-witness who either witnessed the incident itself (not always possible), knew the victims, the perpetrator and (just as important) a person who maybe wasn't associated with the murder, but knew and visited the location around the time of the murder, as it's impossible to get a true sense of what a place looked like from a court document or newspaper article. Through personal testimony I've been able to obtain useful nuggets of information such as how a place looked, sounded and smelled, a perpetrator's nicknames, a person's description, and even their vocal tone. It's the little pieces of information - that most people disgard - which, for a storyteller, is most vital.