I'm writing a story in third person. When writing internal dialog, should I use first person or second person? Does it matter? Should I be consistent in the entire story? Should I be consistent for a given character in the entire story?
Here is the specific example
John strained under the weight of his pack. I'll never make to to the summit. I just can't keep this up.
vs.
John strained under the weight of his pack. You're never going to make it to the summit, Johnny. You can't keep this up.
It's a matter of style. Note that your example is the same: both are from Johnny's perspective, just he's referring to himself in the third person in the second case. Personally, I prefer the first example, as third-person self-reference has always felt a bit awkward to me. I never refer to myself in the third person in my thoughts.
Whichever you decide, you should remain consistent throughout. Inconsistency in any form, be it exposition, dialogue, or your personal writing style, throws your readers off, and can keep them from immersing in the story.
Both First Person and Second Person internal dialogue would be acceptable, however I much prefer the First Person example. I find it more realistic for someone to be thinking "I'm never gonna make it!" than telling himself "you're never gonna make it!".
You could also keep their thoughts in Third Person, although it wouldn't be as personal. For example, "John strained against the weight of his pack. He was never going to make it."
Now, if all of his thoughts were in Second Person I don't think I would like that. However, if it was only one instance where he referred to himself in the Second Person, it could make the moment more intense.
Internal dialogs are very personal events. There's usually only one character involved, barring schizophrenia or telepathy. That being the case, you can easily drop a few pronouns and make the whole matter moot...
Never going to make that summit! Can't keep this up.
