There is a major distinction between just saying the "right words" versus truly believing in the abilities and potential of the person you wish to encourage.

Saying encouraging words only because you know it's the right thing to do, but not believing in what you are saying, will limit the effectiveness of your words. Your negative feelings will come through in your tone of voice, facial expression, and posture, despite your encouraging words.

When you sincerely and authentically believe in someone's abilities and potential, your inner confidence and certainty will be obvious in your tone of voice, facial expression, and posture.