I've just been reading that Avoid Writing a Mary Sue tvtrope article, and there's a point I disagree with and want to discuss with you guys:
Now, if I'm reading this correctly that line expects fanfic writers to go into detail when explaining a canon character if they intend to do the same with an original character, but I disagree with that.
I've read fanfics where they do this and I find myself losing patience reading descriptions on characters I already know what they look like and I'm far more likely to stop reading - especially if they go into detail like what clothes they wear (Solas' turtleneck for example*; I know what it looks like, stop describing it and get on with the story). In my opinion unless there is a key difference in a canon characters appearance that differs from canon (concept!Solas) then physical descriptions for canon characters should be ignored.
That said, I think OC's need detailed descriptions, at least (and usually most) at the start (which this quote argues against) when you first meet them so the readers know what they look like. Being vague about appearances can work for certain characters (ME/DAI protagonists), but a big part of making an OC fit in the world where that's not needs the description.
*Harhar! On topic!
I agree with you.
My philosophy on writing anything really is: show not tell
As in, you have to describe directly when necessary, but always go for showing who your characters are before you tell the reader who they are, and this goes for original OC's especially.
You can't just dump in an OC and be like "welp, that's that, let's move on."
Who is the OC? Why do I want to read about him/her?
In terms of already established characters, description is completely unnecessary imo. We already know who they are. Telling us what we already know is boring and makes us take the fic less seriously.
I think showing who the character is in relation with the main character is how a writer should establish their version of a canon character, rather than describing them incessantly.
As a reference, Feynite in The Looking Glass and The Dread Wolf's Heart does "show not tell" perfectly. It's partially how I think she keeps her readers so invested in her stories