There are a number of extra scenes which are not in the book that I would love to see in the movies. One is the story of how Thorin earned the name Oakenshield, and the other is how Gandalf managed to persuade Thorin to form a secret mission to the Lonely Mountain and take a hobbit with them. Following is the first conversation between Gandalf and Thorin at Bree in March 15th 2941.
'Master Gandalf, I know you only by sight, but now I should be glad to speak with you. For you have often come into my thoughts of late, as if I were bidden to seek you. Indeed I should have done so, if I had known where to find you.'
Gandalf looked at him with wonder: 'That is strange, Thorin Oakenshield,' he said. 'For I have thought of you also; and though I am on my way to The Shire, it was in my mind that is the way also to your halls.'
'Call them so, if you will,' said Thorin. 'They are only poor lodgings in exile. But you would be welcome there, if you would come. For they say that you are wise and know more than any other of what goes on in the world; and I have much on my mind and would be glad of your counsel.'
'I will come,' said Gandalf; 'for I guess that we share one trouble at least. The Dragon of Erebor is on my mind, and I do not think that he will be forgotten by the grandson of Thrór.'
J.R.R. Tolkien. Return of the King Appendix A, III. p. 359.
In "Unfinished Tales" by J.R.R. Tolkien, the interactions between Gandalf and Thorin that led to the "Quest of Erebor" are described in the first person. Gandalf relates the story of how he met Thorin to Frodo, Sam, Merry, Pippin and Gimli after the coronation at Minas Tirith prior to their return to The Shire.
After the meeting at Bree, Gandalf went to Thorin's Halls in the Blue Mountains and heard Thorin's tale. It became obvious to Gandalf that Thorin was agonising over his wrongs, the loss of the treasure of his forefathers and that he had an immense sense of inherited duty to take revenge on Smaug. The sense of duty is taken very seriously by dwarves. (Unfinished Tales: p417). While Gandalf was as eager as Thorin to eradicate Smaug from the Lonely Mountain, he could not agree with Thorin's "lofty designs" to form an army and go into battle to reclaim Erebor "as if he really were King Thorin II". He suspected that an open attack on the Dragon in Erebor would not go unnoticed by Sauron in Dol Guldur and any army Thorin could assemble would have been doomed from the start.
Thorin, being proud, obstinate and with a burning grudge, would not listen to Gandalf's entreaties that an attack on Erebor would be perilous and without hope of victory. He left Thorin without a definite plan or assurance to help him and travelled to The Shire.
Next: Gandalf formulates a plan.