This question may be impossible to answer, but I am asking what Tolkien said about this issue, not what people think about it personally.
In the movies, Smeagol seems to be very different from Gollum. This difference is less concrete in the books (at least up to where I am now - The Passage of the Marshes in The Two Towers). Smeagol, generally speaking, is more human (for lack of a better word), obedient, considerate, and friendly, and at least appears to be less concerned with the Ring than Gollum is; Gollum is sneaky, malicious, dangerous, unpleasant, untrustworthy, and conniving, and cares only about his "precious".
The thought that sparked this question came from the scene in the films where Frodo hands Smeagol over to Faramir (albeit reluctantly and because he has no other choice). Gollum seizes on this to persuade Smeagol that he was right all along, and Frodo really was his enemy. Smeagol resists for a while, but eventually relents, and Gollum comes to the fore once more. This made me wonder what would have happened if Frodo hadn't appeared to betray Smeagol. Now I wonder whether the distinction between Smeagol and Gollum is as firm in the books as it is in the movies.
I am not concerned with which persona is stronger, nor with which persona wins in the end. Even if Smeagol is as trustworthy as he seems to be, surely the Ring will have its way sooner or later.
But for the time being, before the crucial "do or die" moment when they reach Mordor and Gollum/Smeagol is forced to choose between Frodo and the precious, was Smeagol truly different from Gollum? Were his intentions - however conditional and temporary they may have been - good? Of course, Gollum was always going to win in the end, but did Smeagol ever really intend to oppose him?
Did Smeagol ever care about Frodo as anything other than the guy who happened to have the Ring? Did Smeagol resent Gollum as he clearly does in the movies? Did Smeagol - even in part - want the Ring to be destroyed? When Smeagol saves Frodo's life, or protects him, is he doing so solely to protect the Ring, or is there a genuine concern for Frodo himself involved? In the Dead Marshes, when Sam hears Gollum and Smeagol arguing while Frodo sleeps, Smeagol initially refuses - quite adamantly - to even consider the possibility of hurting Frodo, and - surprisingly - even Gollum concurs:
Not hurt the nice hobbit, of course, no, no.
However, Gollum is more than willing to hurt Sam:
Make the other hobbit, the nasty suspicious hobbit, make him crawl, yes, gollum!
Smeagol has no problem with this; rather than objecting to the idea of hurting Sam, he merely checks to make sure Frodo won't be harmed. Gollum agrees that Frodo won't be hurt, then tries to undermine Smeagol's affection for Frodo. He points out that Frodo is a Baggins, and a Baggins stole the precious; even if Frodo wasn't the thief, he never tried to return the stolen property. Gollum insists that all Bagginses are the enemy. Again, Smeagol resists:
No, not this Baggins!
Gollum only gains ground when he changes the subject a bit, and gets Smeagol thinking about how wonderful it would be to have the precious back. He avoids the issue of how they could get the precious without hurting - probably killing - Frodo, and Smeagol takes the bait without noticing this omission. Gollum gently leads Smeagol along, painting pictures in his head of how great in will be to be reunited with the precious. Finally, he drops the bomb - they should bring Frodo and Sam to Shelob's lair and let her deal with the hobbits. Smeagol snaps out of his trance and recoils in horror - he still refuses to contemplate bringing the "nice hobbit" into harm's way. Frustrated, Gollum drops the subject, saying that perhaps it is too soon to kill off the hobbits; there the conversation ends.
So my question is this: Is the difference between Smeagol and Gollum as stark as it seems to be? Is there real conflict here? What did Tolkien say about this subject?