The act of changing voice, body language, movement, gesture etc when in role is called characterisation.
All people are different. The actor must use their skills to portray a character consistently throughout their performance. When creating characters you need to consider the following:
Voice: Does your character have an accent? What is the tone of their voice like? How quickly do they speak? Do they have any vocal mannerisms that are particular to them?
Body language: This is what your character’s movements and way of using their body says about them. A character who is very nervous and stressed may fidget a lot or have their shoulders hunched up tight to indicate tension.
Facial expression: Does your character move their face a lot? What does their facial expression say about their character? Do they have a very expressive face or do they try not to give much of themselves away?
Hot-seating: (being questioned in character) is an excellent way of ensuring that you understand the role you are playing.
Improvisation: in role is also very useful in rehearsal as it ensures that you can act as that character ‘off the text’. It helps you to understand how they would react in a range of circumstances.