Why would someone act one way toward you when it's just the two of you, but totally different when others are present?

I don’t know if this is relevant to the question or not, but it’s a coworker. He acts totally sweet, very nice, etc., towards me when we’re talking one-on-one, but sometimes barely acknowledges my presence around others, particularly higher-ups. I’d be interested in hearing people’s thoughts on possible reasons for this behavior.