There is no right or wrong answer to questions like "What are the most difficult decisions to make?" or "Describe a difficult work situation / project and how you overcame it." These are behavioral interview questions designed to discover how you handled certain situations. The logic behind these types of questions is that how you behaved in the past is a predictor of what you will do in the future.
Give concrete examples of difficult situations that actually happened at work. Then discuss what you did to solve the problem. Keep your answers positive ("Even though it was difficult when Jane Doe quit without notice, we were able to rearrange the department workload to cover the position until a replacement was hired.") and be specific. Itemize what you did and how you did it.
The best way to prepare for questions where you will need to recall events and actions is to refresh your memory and consider some special situations you have dealt with or projects you have worked on. You can use them to help frame responses. Prepare stories that illustrate times when you have successfully solved a difficult situation.
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