How do you keep calm under pressure? Have you ever lost your cool at someone where you regretted your actions?
How you've managed or would manage and handle various situations as a security guard is important information for the interviewer. And how you interact with co-workers and the public on the job is also important. So your answer should be a positive, description of your work style on the job. Give multiple examples of how you've kept your cool in stressful situations and methods that have impacted those successes. Avoid sugarcoating any failures but know in advance which ones to talk about and which to avoid. Talk about the failures you learned from.