Your personality type impacts on what you do. Everyone has a preferred mode of operation that they use. Over time, with some self reflection and life experience, we can regulate our reactions to situations better - overriding our natural instinct.
I have studied the Meyers Briggs personality types in depth and find them extremely accurate for judging a person's reactions and preferences in given situations.
When I was developing my character profiles, I decided to give my MCs Meyers Briggs personality types so I could use my knowledge to accurately portray their actions and reactions.
For example, I made Mishca an ENFP (Extrovert, iNtuitive, Feeling, Perceiving). In layman's terms this means she likes being around people as it energizes her, she prefers the big picture to detail, makes decisions with her heart over her head and is a fly-by-the-seat-of-my-pants kinda girl. However, in times of crisis she reverts to the opposite mode, is withdrawn, focused and rigid - but will explode if pushed.
Knowing her personality type helped me judge if the progression of Mishca's journey was realistic for someone with that personality type - and I found it was. People seem to think she has too cool a head for someone about to have a heart transplant operation, but she turns logical and withdrawn to cope with the stress. After the operation, when her stress settles down, Mishca reverts to her rash happy-go-lucky persona.
If you are interested in using this system to help with your character development then here are some free tools and reference sites:
The first site includes a free mini test. I highly recommend that you take the test, find out your type AND read the material related to your type as a starting point. You can even get friends and family to take the test too to help you understand personality type dynamics.
The last thing a good writer wants is a Mary-Sue character (a boring MC with no flaws) so mapping out their character traits and including their personality type might help you create three dimensional characters that audiences can connect with.