There are lots of ways to make your characters seem believable. Give your character a birthday and then check your character's horoscope to discover hidden personality traits. You need to know all about your characters - their favourite foods, hobbies, the kinds of friends they have, what sort of cars they like, whether or not they've spent time in jail, what smells disgust them and what music they play while having a bath (or, perhaps your character only ever takes showers...)
These exercises will help you get to know your characters better. Have fun!

Ambitions
What is the one thing in the world your character wants to do more than anything else?
What is standing in your character's way?
If your character could take a trip anywhere in the world, where would it be? What means of transportation would your character prefer? Describe a trip your character takes alone... then describe the same trip as if he or she is joined by the farmer shown below (scroll down).

This man believes these eggs have magical powers. He travels to the local village to try and sell the eggs to a local magic-items trader. The trader disappears into the back room to 'assess the product.' The trader comes back a few minutes later with some regular chicken eggs and tells the old farmer he is crazy, that his eggs are completely normal and good only for making omlettes. The old man is furious because he knows the old magic-trader has stolen his magic eggs.
1. Write down the conversation the old man has with the local police officer when he reports the theft.
2. Describe the phone conversation he has with his daughter, a retired lawyer who thinks her father should be in an old age home.
3. Describe the conversation the old man has with the little girl in the Christmas tree photo above.
4. Describe the conversation that occurs when the old man returns to the magic-trader to deman his eggs back.
5. Describe the interview that airs on the 6 o'clock news.

Dreams
What did your character dream about last night?
Describe the worst nightmare you have ever had.
Let your character fall asleep in an unusual place.
Describe a situation where somebody wakes up in the middle of the night with a brilliant idea. What happens?

Let your character visit a miniature horse farm. Describe the way your character feels when he or she is surrounded by a herd of little horses. Remember to describe the scene using all five senses.

This family is out buying a Christmas tree. Just after this picture is taken, a boy runs towards the family shouting, "That's my tree!" A big argument follows and somehow the boy gets a bloody nose.
Describe what happened from the point of view of the little girl.
Describe the same scene from the point of view of the boy with the bleeding nose.
Describe the same scene from the point of view of the mother. And then, do the same thing from the father's perspective.

Describe in detail this boy's room at home.

One of the people in this photograph has a terminal illness. Do the others know? If so, how does each feel? If not, why not? Describe a scene where these three people go out together and the sick one collapses. Try describing this event in the first person and then again in the third person. Describe the collapse from the perspectives of at least two different characters.
Repeat the exercise but choose a different character to be ill.