We had some friends over last night, and I observed something very interesting about my husband that made me think about our recent accent discussion here: his accent changes depending on context.
You may have noticed this about yourself, or others, already. Does your friend who grew up in New Jersey sound different when she talks on the phone to her relatives back home? Does your Southern buddy twang more when he gets around others from his region? This stuff happens all the time.
My husband, as I've previously noted, has a halfway-accent after his fifteen or so years here. Aussies think he sounds American, and Americans think he sounds Australian. When we go back to Australia, or when he talks to his mother, his accent starts to gravitate unconsciously back toward the Australian norm.
However, when he intentionally "puts on" an Aussie accent, it's not his natural one; it tends to come out exaggerated, and often he clicks at the end of what he says. The clicks have always surprised me - they're the kind in the side of the cheek that people make to get horses to move. Since I only noticed them recently, I'm not really sure whether they have some basis in a local Australian accent, or in someone's idea of the "ocker" Australian accent, or even in some comedy routine. If you're Australian and/or you have any clue about this, I'd be curious to hear it.
The other time that his accent gets stronger is when he's telling jokes - it's clearly not put on, but unconscious, and yet it's a significantly stronger accent than he gets when speaking to his relatives.
I think there's got to be some great application for this in a story, so I'm going to be looking for a place to use it. Maybe someone undercover who gives himself away by joking, or losing his accent in a critical situation... You can keep your eye out for it, too.