The last post in The Actor Game got me all hot for this one, Agent Milton Dammers from The Frighteners:
He plays a Neurotic twitchy detective who, while doing what he thinks is good, does not know what is really happening and continues to persue Micheal J. Fox who is the one actually trying to stop the murders in his town of fairwater.
This is a great scene, albeit a few spoilers. I suggest watching the whole movie though: