Read the script several times. Look for specific mentions of your character's age, likes, dislikes, family history, hobbies and background. Circle or highlight each of these sections. Be careful when reading to look for areas of sarcasm, lies and other trickery that may make something they seem to like on the surface actually something they depise.
Write a series of questions on a sheet of paper. These should include age, place of birth, place of residence, occupation, social status, siblings, parents, habits, flaws, skills, hobbies, dreams and fears. There are more items you can write down as well but this a starting place. Write the answers to all these questions. If you didn't find the answer within the script then create it for yourself with your imagination.
Write each major and supporting character in the script on a separate piece of paper. Write a sentence or two about who they are (their relationship to you) as well as how you feel about them. Finally write what it is you want from each and every character. Everyone wants something even if it's just having someone to talk to.
Read the script again. Figure out what your purpose is in the story. Are you the main character and the main goal is yours? Are you a minor character that foils the hero's plan? What is your purpose? Secondly, figure out what your motivation is. What makes you do the things you do? Why do you want whatever it is you seek?
Combine all the written information and the motivations and purpose into a single paragraph description. Start with your character's name, age and occupation then drift into their purpose, goal and motivations. Within these senteces fill in background details to create a full person.