Part of what you described sounds a bit like mirroring. (The part where you said you tend to act like the person with whom you're talking.) I think we all do this, in some form, whether we're aware of it or not. When we're talking to someone, we sometimes unconsciously mirror that person's body language, mannerisms, or speech. The idea is that doing so will help us build a rapport with the other person.
I think some people are more disposed to mirroring than others. Maybe you're one of the people who is more disposed to it?
Anyway, it doesn't mean you're insane. Just don't lose sight of the person you are when no one else is around.
[MENTION=23956]Mychemicalkilljoy[/MENTION]
Just to help out with this mirroring thing. Someone might be insane for example if someone asked them how work was and then the person responded with extreme anger and started screaming for help.
When our mirroring is messed up we think that dangerous people are nice and good people bad.
This is also why drugs are bad.
Of course, mirroring is also the primary mechanism by which a psychopath can hack into our psyche or fall in by way of Trojan horse.
Mirroring is a thing that can be learned and is not necessarily as genetic as much as environment for most people.
But to be entirely fair, most psychopaths are actually quite naive because no one knows for sure how mirroring works both from the inside and outside.
We can only really approach this thing as if it were a novely for the idea of mirroring as a neurological phenomena is relatively new and from the outside it's a matter of intuition and experience.
But mirroring is deep and someone witha deep intuition will know when someone is messing with them, or mimicking them...but go ahead and experiment.
Try sitting, looking, and doing the same things the other person is but inverted like a mirror and please do let us know how it goes.