The name game's a tough call in writing: deciding and re-deciding as we go along what to call a character.
I've read books by some very fine writers and watched them wrestle with the same conundrum. On one page--or in one paragraph--they may call a character by his full name, his first name and his surname. I understand the drive to do this. The inner ear is at work and sending out a signal that the repetition of names is too much and variety is required.
Or, possibly, the author is sending out a very subtle signal that this character is shadowy or fluid.
Similarly, if one character--let's say a very tough one--has been called by his last name throughout...then in a vulnerable moment is referred to by his first name, the new sense of intimacy can take us by surprise. And it's generally a wise idea to get the character's full name out upfront.
That said, I found myself doing a three-name as I typed the first draft of my new fall thriller. Boss, the hero, calls the villain Robert Johnston...then RJ...then Johnston. Sometimes he refers to him by his position: majordomo. I could chalk this up as boredom with name repetition. But something else may be at play: either Boss or I have failed to form a solid take on RJ...or Robert...or Johnston...or the domo.
We'll see. The most likely solution, at this point, may be: Introduce him as the don's majordomo, Robert Johnston...have him tell Boss, please, just call me RJ...have Boss think/write of him as Johnston or the domo...then, at a critical moment, address him as "Robert".
Somehow I'll contrive a way to wed my instincts and my inner ear.