Speaking from personal experience.
I find it much easier to sell myself when I:
A) Already know the person I'm talking to.
B) Am incredibly happy, motivated and everything's going my way.
C) Have supportive people in my life who don't constantly yell at me, who accept me for who I am and who aren't under extreme stress.
D) Have practiced and go into the thing with an open mind and back-up plans.
In such a happy state of mind, I don't mind bending the truth slightly, because it is closer to the real truth itself.
However, if someone asks me a question about a course I've been on that I really didn't enjoy because I felt socially left and or was messed about by the admin people or became really depressed, I clam up.
When I'm stressed or depressed, and receive no sensible or practical advice from anyone, selling myself becomes almost impossible.
It feels like I'm living a lie.
Why should I say that I'm happy when that's not true?
If I was really happy, I would be more than happy to say it!
I read about a Buddist teaching that said something along the lines of:
"To thine self be true."
And talking to everyone the same way regardless of rank.
This was from a self help book that someone had lent me (he obviously didn't know exactly what they'd given me, but had meant well at the time )
It didn't mention anything about interviews though.