Read your script and take written notes of your character's personality traits. Many actors see visions of the character as they read through the script; these are clues for you to draw on to help develop a convincing personality and to immerse yourself in the character.
Read your notes. Pick out a key character attribute and practice using it. For example, if your character is incredibly happy, practice being incredibly happy while saying your lines.
Take note of any mannerisms you intend to use as you practice your character's key attributes. For example, when acting incredibly happy, how are your eyebrows formed? This will help you be in character when the camera is rolling or the curtains open.
Keep the notes that are not key attributes and let them develop or evolve into your character as you rehearse your role. For example, if you note that you have a high-status character, you may change your posture to be more erect. Continue to memorize and rehearse your lines until you have thoroughly mastered your part and practiced every attribute that you noted.
Clear your mind completely of any encumbrances of your everyday life. Take time off work or school, if necessary, so that you will be able to perform with a clean mental slate.
Act as your character in everyday life. For example, for the film "Taxi Driver," Robert De Niro became a New York taxi driver for several weeks before filming. This helped him to understand his character and to change his personality completely.