I've recently started working through my own series. This is my first really intentionally plotted series with characters that will carry over into around seven books. There's a whole different tactic to creating their stories and personalities than for one novel. It becomes the trick of putting in enough to keep the readers intrigued and have a good knowledge of the character without everything being revealed in one book. Reveal it all at the beginning at there won't be anything left to get a reaction from your readers.
Every day when I sit down to plot, develop, create, or write, there are steps that I take to evolve the characters. It generally involves looking up some type of personal journal prompts and having them answer those questions. 1. It puts me in the mind frame of the character. 2. Some questions that did not occur to me are answered. 3. This furthers the plot and creates fresh inspiration for my writing.
I also incorporate growth into my characters, through applying or releasing pressure on them. Stress, fear, and pain can reveal completely different aspects of a person's character. Conversely, it can show another side of a person who is finally relieved of one or all of those issues. A perfect example would be Mr. Dorrit and some of his children in Charles Dicken's Little Dorrit. There wasn't a complete change, that's not realistic, but small issues became larger, blatant, unmistakable. It causes the watcher to cringe and feel sympathy for Amy Dorrit. On the flip side, you could look at A Little Princess (not a true riches to rags since it all works out in the end). Her dreaming comes to an end for a while because of the lack of material items, but returns stronger than ever because of the influences in her life.
Consider your own growth. You don't remain a robot without change. I used to hate the texture and taste of cheesecake, now it is an enjoyable dessert. Or my favorite animal was a horse, now it is a cat. That's more a change of preferences, but drawing attention to it can save you from personality change while working on development. As I've aged, I've become more of a introvert and indoorsy person. Change can be brought about by an event, person, and change of location.
Music, books, movies, and real life are an excellent source of inspiration. Especially the fictional. Fictional characters are often designed to exaggerate character traits to draw your attention to them. But in the end they are based on real people in some respect.
Don't forget to bring in advice from friends if you are struggling. My own problem is the crafting the female main character. I regularly talk with my female friends to gather insight about how to design a well rounded female character.