David Hare addresses one aspect of this in his BAFTA Guru Screenwriters interview when he talks about "Who is the third who always walks beside you?"--one valid function for a character in a scene is to be a third party complicator who makes the scene more difficult for your protagonist.

Shaula, thanks for setting up another question in the series this week about conflict in scenes. Those BAFTA series are great. I've featured many of the lectures on the blog.

You're welcome! I love this series of posts, Scott. It's easy for me to look at a scene and fall into the trap of seeing it from the protagonist's point of view--but sometimes serving the scene and serving the story is about obstructing the protagonist, too. In that way, I find Hare's question helpful. I keep going back and reading those BAFTA lectures--no matter what I'm working on, I find something that speaks to the writing task at hand. PS Two other good related links: Bill Martell on writing conflict and Beth Hill on conflict.

One writer I once heard speak was a big fan of having a third character in a two person scene: an observer, someone whose reactions to things provides a model for the audience's reactions. Which is an interesting idea...

PS Here's another good scene question from David Hare, from the same interview: "Are your scenes only about themselves?"