I always recommend having two stories for each of the areas of leadership, taking the initiative and working with difficult people. These stories should be presented in the STAR format (situation, task, action, result) and be about 4 mins long.
The McKinsey FIT interviews are longer (typically 15 mins) as they ask you very in-depth questions about your story - you’re not expected to just present for 15 mins! I recommend having 2 stories for each area in case for some reason your favorite story doesn’t answer exactly what is asked.
You then want to be able to answer the standard questions of tell me about yourself, why consulting, why this firm, what are you proud of. These questions tend to not get asked by McKinsey but often are by other firms.
Good luck!