When coaching clients - I often get asked how to go about explaining why they quit their last job. Sometimes the explanation is simple: relocation or layoff. However if you left because you couldn't stand your boss and weren't treated well, that's a little bit more difficult to explain. It's best to always describe the circumstances in a positive manner, no one wants to listen to you bash your previous employer during an interview. Cater your answer to your current situation. For instance if your prior company was small and privately held and your company your interviewing for is public, you can simply say you want to work for a larger organization with the growth potential like XYZ company. If you are interviewing for a position that differs from your last, pick out the key duties of the potential position that were non existent in your last position and explain that you are looking to add those skills.
Remember, always keep it positive when explaining your reasons for leaving.

"Explaining Why You Quit Your Last Job" Employment Digest Becky Mease

Many times during an interview, the hiring manager will ask that dreaded question most hate to answer, “Why did you leave your last job?” Not all answers are going to be as easy as, “The Company was down-sizing and I was unfortunate enough to feel the ax.” Many folks leave a job for not receiving promised outcomes, for have personality clashes with employers and/or co-workers, lack of company vision, and even for feeling undervalued. The entire situation may become even bigger if you quit and didn’t give the customary two weeks notice. However, there is a productive way to explain your personal situation that will still have you looking like a professional.

When an employee feels like they are being abused by their boss or are undervalued, it’ll take that one single moment where they’ve decide they can’t take anymore and then just up and quit. No notice, no explanation. At the time, they gave little thought to how they would explain their leaving at their next interview. However, just because they left rather promptly, doesn’t mean it could cost them any future employment.

The best advice is to not bring it up at all if they do not ask you. Most hiring managers will- but not all. If they do, you will have to have a well-crafted answer that will have the hiring manager empathizing with your situation and believing what you did was due to no other alternative. Never should you cast stones at a prior employer- your explanation should be truthful and very general. Employers don’t like to see blame shadowed over governing entities- and there is always two sides of a story… and yours can be checked. Taking responsibility for problems looks more appealing to a prospective employer than listening to a candidate belittle past bosses.

If you are asked why you left, instead of telling the hiring manager that your boss was a closed-minded jerk, you might say you and he had very different work styles. So much so you had to quit. Of course, they’re going to ask you to elaborate, so go further in saying that other bosses you’ve worked under allowed for more responsibility and independence in your work. While your latest boss approached projects differently, and had you feeling your work was undervalued and lacking. Again, take some responsibility for the reason you left- concede to the fact that you couldn't’t handle your bosses work style. This is more product than kicking your old boss around leaving the hiring manager questioning your professionalism.

We have found that many interviewing questions today are more open-ended that require and interviewee to elaborate on situations that had occurred in previous employment environments. This type of questioning is more behavioral in terms where they look to see how a candidate reacted or would react to certain situations. You’ll almost always hear the question, “Tell us about a time in your previous employment history where you had a problem with a boss, co-worker, or the job itself.” They’ll then ask for to further explain what you did to solve the problem. Again, it’s all in the wording and how you make yourself look while still maintaining your professionalism.