Mostly this is preceded or followed by a statement that goes something like this: "I would never say this on an interview, but I wanted to tell you the truth."
Words and positioning are everything. So while I encourage the truth, once said, it becomes part of what is used to form an opinion of a candidate.
The candidates I talk to almost always are in the business of selling and marketing. They spend hours tweaking power points, refining speeches and searching for the right words or the best positioning. They rehearse their pitches before going to see the client, they give speeches while looking at themselves in the mirror. I wonder, why not use the same care when it comes to talking about themselves?
Recently, I asked a candidate why he left one job for another one. "I was really bored working on the same business" was the answer I got. Ouch! What if he had said, "I wasn't feeling challenged." Or, "I was looking for a new challenge." I made the suggestion he might consider positioning his decision that way in the future. But as far as I was concerned, it was something that had already gone into forming my opinion of him.
How about if you've been fired or "agreed to disagree"?
Try to explain it in a sentence. Use two sentences if you really feed the need. Any more and you start saying things you shouldn't. Candidates have shared excruciating detail on failed strategies, poor management and unreasonable goals -- all from the boss who fired them. The more information I have, the more questions it raises about the candidate.
What do you say when it comes to money?
This is an area where there's a tendency to fib, especially for more junior candidates. Here's a tale to keep in mind. A few years back, a candidate gave me their base salary. It didn't square with their level of experience or with the norm for their employer. They didn't get the job, because my client didn't think they were worth the salary in comparison to other candidates.
Not long after that, I had an occasion to talk to their boss. I asked what the salary range was of people working for them. My "fibber" had added $20,000. Fast-forward three years; the "fibber" is again in the job market. No need for me to confront them, but I'm going to always question their integrity going forward.
It's just much better to say something like: "I would need at least (fill in the blanks) to make a change." Or, if it's too early to talk about salary, deflect the discussion with: "I really want to understand if it's the right opportunity first."