4.10.14 LMD on the Radio in Oz, celebrate with a cup of tea!

Exactly.  I would hate to think there's a Ted Bundy lurking inside of Mark Harmon.

Of course not. But that would be oversimplifying things, not only acting, but human nature. If I had a penny each time I saw an actor roll their eyes at the question: How much of the role is in you, or vice versa... The answer is all of it... You pull at your dark obsessions, passive corners of your psyche, inactive traits of your character, you dig it up and put it in the front. Modified, amplified, but still a part of you. That's at least how I imagine how all the big actors do it. 
For example, if you only stay on the surface, and take inspiration just from the outside, focus on the physicality of it, you are going to get correct image, but bad acting. Johnny Depp's creation of Jack Sparrow was based on Keith Richards, and you can mimic the appearance, the gestures, the posture, but it would remain a simple gif material unless you breathe life into it, and that life comes from inside of you... That role maybe wasn't the best example for my point, but there are so many to choose from. Marion Cotillard as Edit, P.S.Hoffman as Capote, Bardem as Anton... They all have it inside of them, all the traits and the emotions needed to portray the role. 
If you are acting a whore, you don't have to be one to act it, but you have to have the sexuality, the understanding, the comprehension of the character in order to be able to defend it and to make it believable. Basic drives and motives are all latent inside of us, dormant until we decide to pull them to the surface. The complexity of human psyche is such that what you see is what I want you to see, what I want to present myself like, and there should be no conclusions drawn that it is all of me, or only version of me.
I believe that if Adam was not endearing himself, he could not have acted an endearing person. That trait could have been a less visible, or completely invisible; it is simply a coincidence that we knew Adam as such. I am positive now that Adam's spectre is so wide that he could play many different characters. Of course, that is only a prerequisite, in my opinion, and not a guarantee that the role would be successful.
Also, they don't always have to be inherent emotions or instincts which you draw from, it is also all your history, your experience, education, books, songs, movies, everything that you know and that has left a trace in you... In that sense, I stand by my remark that you cannot act something which isn't a part of you, in some way. 

Oh, thank you so much for the playlist.  I love Adam in Glee.  And I agree about the acting -- I think we all have many personalities within us.  A good actor can draw them out and make the viewer believe.