Let's start with what you shouldn't say (or, if you're the interviewer, what are definite red flags).

Don't talk about how your boss is difficult. Don't talk about how you can't get along with other employees. Don't bad-mouth your company.

Instead, focus on the positives a move will bring. Talk about what you want to achieve. Talk about what you want to learn. Talk about ways you want to grow, about things you want to accomplish; explain how a move will be great for you and for your new company.

Complaining about your current employer is a little like people who gossip: If you're willing to speak badly of someone else, you'll probably do the same to me.