OC Stand In

An OC Stand In is a term for when an author uses an existing Canon Character that gets little screentime or few (if any) lines, and therefore can be molded into whoever the fans want or whatever is necessary for the story. They may sometimes be background characters or involve extrapolating ideas from a brief on-screen appearance. OC Stand Ins can be like original characters, but they come pre-packaged with at least some minor characteristics, giving the author some general direction to go in. 
It is important to note that when done well, this is not a bad thing, as character development is necessary to the inclusion of any character in a story, and even popular canon characters can appear in different forms in fanworks. This can be an acceptable alternate to introducing an original character and sideline 'New Kid' storylines that easily alienate readers. A particularly successful interpretation of one of these can ascend into Fanon when other fans want to reuse the OC Stand In as developed by another author, such as the popularity of Gregory and The Mole.
Sometimes these characters are just used as under-developed antagonists, making them somewhat pointless. In these cases, the character is simply used to replace the canon obstacles for the protagonist with smaller and (sometimes) more realistic ones. There are also some rare cases in which these characters can become a Mary Sue.
Category: Terminology