Preparing for a second round Interview

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I once went for a job interview and was not fortunate to make it to the second round. When i probed the manager who interviewed me on that day he told me my skills were proficient and excellent and what had failed me was that my assertive skill needed an upgrade. I recently went for another interview for the post of team leader. What measures should i keep in mind before my interview day so that i can showcase more assertiveness to the interviewer and also potray a positive body language?