How can I make a specific file in my directory hidden or read-only?

This is not possible for directories in AFS. AFS permissions take precedence over permissions set by the "chmod" command.

While you can make a file read-only (with "chmod a-w filename" or similar) to prevent inadvertent overwriting of the file, anyone with AFS "write" permissions would be able to run "chmod a+w filename" and overwrite it.

If, however, you have a directory that is shared with your friends, and you want to prevent them from reading certain files in that directory, those files must be moved to a different directory on which your friends do not have any permissions.