Keep it short
Respect users’ time. Get to the point and get out of the way.
Give them credit
People know the language and they know how to talk, so avoid telling them how to speak and putting words in their mouth. Instead focus on natural ways to progress the dialog forward.
Optimize for relevance
Be contextually relevant and sensitive to the user’s current need and the environment a user might be in.
Delight the ear without distracting the mind
When adding personality, be sure it’s not so over the top as to interfere with a user’s task.
Engage beginners and attract experts
Designing for many people doesn’t mean designing for the lowest common denominator.
Don’t take back your turn prematurely. If you ask the user a question (yield your turn), don’t throw in an additional instruction that gets in the way of their right to answer you.
Don't read minds
Give them the facts and let them decide.