The original quote goes:
> "Courage is not the absence of fear, but action in the presence of fear."
It appears that the quote you are looking for has been slightly misremembered or altered.
It is possible that someone, perhaps even a character named Ambrose Redmoon in a fictional work, has used a similar phrase inspired by Twain's quote. But the quote itself does not appear in any known movie or work by Twain.