When I read a book with riveting characters and the sequel finally hits the market, reading book two often feels like I am meeting a group of old friends again after a long interlude.
Here is a tip I use: When developing my characters, I write about them like I know them. In other words, I imagine the people in my book have been long time friends, or enemies, and I know them well enough to help my readers know them.
How can you write about a character if you have never met them? It's time for you to meet your characters! Interview them. Talk to them. Ask them questions. Talk to their friends and family. Ask their co-workers and boss questions. Get to know your character so your readers can know them too.