No matter how negative a past work situation may have been, no matter how bad a former boss may have treated you, you should always carefully avoid badmouthing the firm or anyone in it. There’s no percentage in getting negative. Always keep the entire interview cycle as positive as possible.

When you stop and think about it, being positive is key in any field. When was the last time you heard a movie star say "The director on that film was a real jerk! I hate that bastard!" Or when did you last hear a politician say something rude about a political opponent? CEOs of large corporations are universally positive even when asked for their opinion of business competitors.