I have a first draft of a novel set in Cornwall, which is about three-quarters complete, and when I finish this draft of Lives Interrupted and send it out into the world I will get back to the Cornish novel (as I call it because it's still untitled).
I mention this because I worked hard on the characters for the Cornish novel before starting to write. When I first came up with the initiating incident I knew that character was going to be a huge part in what happened.
The main characters are siblings: two brothers and a sister, and central to the story is their relationship with each other, how they view themselves, and how their place in the family structure i.e. oldest, youngest, middle child, impacted on them.
One thing I found incredibly useful was Natal Charts, or birth charts, as they are sometimes called.
Astrological signs are a useful beginning point for a character, but natal charts also look at rising signs, moon signs etc as well as the sun sign. This gives more in-depth character information, and shows positive and negative sides of character traits.
I selected a sign for each of my main characters that reflected my initial impressions of them, and from that chose a birth date for each of them. There are numerous websites that have this information and give free information from a birth date and place.