just watched the hobbit 2 movie and it's a big question for me! what was the plan? to kill the dragon or just to find the arkenstone? it seems that they didn't want to kill the dragon because after finding the arkenstone they were trying to escape! what they wanted to do with the arkenstone?
In the movie adaptation, the importance of the Arkenstone was vastly expanded, compared to the book where it only represents the most precious item of a vast treasure, and is considered as a share all by itself:
In Jackson’s world, the Arkenstone holds global significance for all of the dwarves—not just Durin’s Folk. Whoever possesses the Arkenstone automatically becomes their ruler. “The Arkenstone is not supposed to be a mechanism,” Drout complains. “They’ve taken it and turned it into the Ring.”
The plan in the movie is then more or less, "Since the other dwarves refused to help us, let's try to go to the mountain anyway, steal the Arkenstone (hence the need for Bilbo), and therefore give Thorin the authority to call a big army composed of all dwarves to take out Smaug for good, instead of only requesting it"