Question: I have my characters formed but just their names and birthdays and some aspects of their personalities but I want to know them more. How can I know them more? What questions can I ask to know them better and make them believable and strong?
There are probably an infinite number of questions you could ask about your characters, just as with real life people, and only you will be able to find the most relevant questions. However there are some general areas worth addressing...
1. External. These are questions about the character's appearance, gender, job, hobbies, family, speech patterns, social class, and other aspects of their life an outsider would see from a distance.
2. Internal. These are questions related to the character's personality. In Dramatica, they fall into 4 subcategories:
a) Purposes. What is the character trying to achieve? What does he/she want in life? What would satisfy him?
b) Methods. How does the character try to achieve his goals? What is his/her typical approach to solving problems? How does he/she react to unexpected challenges and situations?
c) Evaluations. How does the character judge things? What does he/she think about the world and other people? How does the character judge himself? How does he decide if he's winning? What are his beliefs?
d) Motivations. I have addressed these in the article on Archetypal Characters in the How to Write a Novel section. Take a look there.
3. History. Here's where you ask why a character is the way he/she is. What significant events happened to the character in the past that shaped his personality?
Remember too that to make your characters believable, you must make them consistent. That doesn't mean they cannot change, but it takes a tough situation to make a person change, and even then they generally can only change one aspect of their personality.
As for strength, readers generally see a character as strong if he or she makes the "right" choices even if it costs him heavily. Of course, what's right to one character may seem wrong to another.