I recently had occasion to want the archive templates for two categories to look different. I must admit that upon thinking about it, this can’t come up too often, but I was working on a client’s website and he has a News category, where he can post updates about Mercedes-Benz (he is a sales representative at Mercedes-Benz of Tampa) and he also has a Feedback category, where people can leave feedback on his site, which are turned into posts in the feedback category, that people can then read.
In this instance, I wanted to put a little extra text at the top of the feedback category, saying that the feedback was left by clients of Simon and you are welcome to leave feedback of your own using the feedback form.
So I did a little research and found that this need is supported natively by WordPress and is very easy to implement. Using FTP, go in to your active theme files (normally located under wp-content/themes/themename/) and you will likely find either category.php or archive.php. Assuming you want to work from the category template you already have, make a copy of the file and call it category-slug.php, where slug is the category slug. If you’re not sure what the slug is, go to your Categories menu under the Posts menu and you’ll be able to see the slug of the category you want to modify. Make sure that you keep the file in your active theme’s directory (where you found the original category.php/archive.php).
Now you can edit this new file to achieve the modifications you’re looking to make (see How To Edit Your Theme Files). In my case, it was placing some text before the list of posts and removing the comments call (as the posts in this category are not open to comments).
A similar method can be used for creating unique author pages. You just need to copy author.php and rename it author-ID.php where ID is the author ID. There is more information on how exactly this works for categories and authors in the Codex.