This is why exercise is needed. I believe that practice is fundamental for the solidification of a virtuous character. I still fail to see how people could still be considered possessors of virtue if they do not apply it (the intentionality factor is a key one here). Besides being a manifestation of the good, virtue is also a principle of temperance and moderation. Therefore a person who is courageous for example, demonstrates that he is half way between a reckless behavior and one which could suggest indifference or cowardice. Virtue can be opposed not only to non-virtue, but also to passions. Perhaps it would be more wisely said that it is the passions which are more likely to lead you in the direction on injustice and unjust acts. Moderation...

Under this circumstances it becomes clear why the concept of moderation is so important.
To conclude with, Aristotle believes that moral virtue can be taught and that the virtuous character of a person is built through constant exercise of the virtues (moderation playing a fundamental part in the process). The philosopher declares that the possession of virtue enough is proof of the virtuous character of a person. Nevertheless, I believe this specific aspect is not clear and is subject to further debate.

Bibliography:

Aristotle (Ross, W.D. Translator). Nicomachean Ethics. World Library Classics, 2009