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It helps in getting a break but it is not a precondition. The two urchins who worked in Slumdog Millionaire actually came from a slum colony but qualified themselves for an aware on their very first performance. That they were asked to behave as they usually do does help draw out the best in them. One should acquire what the Yogis knew as 'parkayapravesh'(entering the body of a different person) and entirely forget their original personality which helps give your best to the role.