What can an actor use to make their character believable and portray the character they play better , even if the character is the total opposite of them ?

Physical exercises can be useful here. For example: Think about what type of animal your character might be, and move around as that animal. How has your breathing altered? What part of your body are you leading with? Are you a bold or timid creature? Do you take up a lot of space or very little space when you move? Do you like to be at the centre of things, or do you stay near the edges or corners of a space? All those things and more can then be drawn into your character when you return to human form!