The Critic wrote:

Hi all.

I've decided to post small 'tricks of the writing trade' every once in a
while to help unexperienced writers polish their craft.

Canonical Characterizations 101:  Descriptions

Many unexperienced writers feel the need to explain facets of a
character we already know (canonical characters, not new characters whom
we naturally know nothing about). This practice clutters the prose, it
can make the reader annoyed when the material contradicts his own
perceptions of the character, and it's often taken by experienced
readers, such as myself, as a set-up for an OOC description.

In a nutshell--don't do it. You know how a character looks, acts and
what is capable of, and so does the reader. It's acceptable only when
done through the eyes of a new character who doesn't know anything about
the canonical characters, and only then when it isn't gratuitous.

Naturally I disagree.  Spending a little time describing on character 
description is a very good idea if one bears in mind that not everyone
reading the fanfic is totally familiar with the source material. 

However, personality should ideally not be described, but 
rather demonstrated.