Now that you know all about the 16 MBTI personality types, what next? These blog posts have really been a way for me to organize my thoughts, so I'll tell you what I did next.
There are 10 main characters in my current WIP. I typed each character according to their general (and sometimes more specific) traits. I already knew what role each played and some idea of how they related to the main character. This all came about through my plot outlines (I generally come up with plot before character, other than perhaps the main character).
To type a character you don't know too well yet, go back to the general descriptions of each trait. Is this character a people-person, or do they spend a lot of time alone? Do they live for the moment or have a vision of the future? Do they make decisions from the head or the heart? Are they spontaneous or do they like firm plans?
This gives you the four-letter type. Next, read the description of that type and see if it matches your character. If it does, great. For example, the hero (read "love interest" if this were a romance, but as you know I don't write romance, no siree) in my story is a scientist. That makes him an intuitive (N) thinker (T), and probably introverted (I) because he spends a lot of time in his own head. The last letter - P or J - was easy, too. This guy doesn't think much of titles, authority, or pointless things like social customs. That makes him P. INTP is the "architect" or "engineer" type, and the description fits him perfectly. He's curious about how the world works. He trusts logic over opinion. He may appear aloof or oblivious, but he's just busy listening, concentration, and taking in information. He's a little disconnected from people but can be charming and witty too.
On the other hand, if the description doesn't match your perception of the character, or what you want/need that character to be, look closely at which trait you're having difficulty with. Reverse it and read that type instead to try it on for size. For example, I typed a second male character in my story as ESFP. I knew he was a people-person (E) who's more interested in the here and now (S) than in theories and possibilities. I knew he was a bit of a rebel (P). I picked Feeling (F), to be honest, because I wanted him to be different in that regard to the first guy above, who's (T). But the ESFP description just didn't feel right. ESFP is concerned purely with having fun, living life to the fullest and having lots of friends to share new experiences with. ESTP, on the other hand, while seeking the best in life and wanting to share it, is also interested in ideas and problem solving. And ESTP is the persuader - the best of all the types at influencing others. This is important in terms of his relationship with my heroine. So this is the template by which I'm writing him. Meanwhile, the ESFP type is perfect for the heroine's fun-loving but irresponsible brother.
As a final example, a key character in the story is the heroine's grandmother. She has a small, off-screen role that has a huge impact on the story. I found myself typing her as a stereotypical wise grandmother type - ENFJ, the teacher/mentor. ENFJ is all about helping people be the best they can be, being warm and supportive, and strengthening social connections. That's lovely, but this woman is also a highly respected starship captain. I just wasn't sure this type would work in the story, even though each individual letter seemed to apply: she's outgoing, intuitive, cares about people, and likes structure and tradition. The solution, unfortunately, was to tone down her caring side. After all, a captain can't be too concerned with the feelings of her crew when she's making life-and-death decisions based on the cold hard data. But that doesn't mean this grandmother extraordinaire isn't ENFJ. It just means that she has a tough time making the necessarily brutal, rational decisions a captain must make.