So, over on the other "Structural Reason for Villains..." thread, I stumbled into a discussion about how a villain is the IC as Antagonist.
* Antagonist: An archetypal character who is in every way opposed to the Protagonist
* Contagonist: an Archetype representing the motivations of temptation and hinder
* Traditional Villain: IC assigned with antagonist elements
Things get less clear when you start to create complex characters and divi up the antagonist elements amongst characters.
For the sake of simplicity, let's stick to the archetypes. When an antagonist is not the IC, that leaves the role to the OS throughline to pick up (I'm leaving it out of the RS all together because that just seems too strange to have the RS be assigned as the antagonist. Please do correct me on that if it's been done or can be done, preferably with story examples).
Thinking about a story whose antagonist is not the IC, The Sixth Sense comes to mind. Cole is not the antagonist and nor is Malcolm - it seems to me. Hence, The Sixth Sense doesn't have a traditional villain. The guy who murdered Malcolm seems to be the antagonist - and he is an OS throughline expression.
Other examples are in Romance stories. The contagonist is often a temptation - an old flame that comes back into the picture, a highly attractive person tempting the Protag away from the IC. So, who usually gets the role of the antagonist? A parent, as an example, who is preventing the relationship or marriage from happening - which I tend to think comes from the OS.