Remember: if you get the job, the person interviewing you may one day be your previous boss. The last thing they want is to hire someone who they know is going to badmouth them some day. Instead of focusing on any negative incident, try to think about any positive thing you learned from your previous boss or a way that you say him positively impact things at work.
"My last boss taught me the importance of time management. He didn't pull any punches, and was extremely deadline-driven. His no-nonsense attitude pushed me to work harder, and manage my case loads way more efficiently."
"My previous employer was a very kind-hearted person. Even when things seemed stressful at work or we had a really hectic day, she always seemed to keep her cool. Her attitude set the pace for our whole team."
"I have learned a lot over the years. I would like to think that I have learned lessons from my previous employers. I had one boss who was one of the most giving people and I learned the feeling of being able to give to others. I also worked with another employer who was very professional and 'matter of fact.' I learned from him how to prioritize my day and work efficiently."