Boy Jacob can interact with objects and other people besides Hurley.
Ghost Jacob cannot interact with objects and can only appear to Hurley.
Why You ask? Because the metaphysical make-up of a ghost consists of negatively charged antimatter. That makes them unable to interact with objects. Some people have the ability to see negatively charged antimatter and therefore the being can appear to them, and only them. This characteristic of ghosts allows them to pass through existance in a way that appears they appear and disappear, but it is really just them passing through the thin fabric of spacetime. As for Boy Jacob, he is a reincarnated spirit. He CAN interact with objects because his metaphysical make-up is largely comprised of Gindlerods. Gindlerods are a very rare mutant strain of atoms that allow someone to have the abilities of Ghosts, and still be able to interact with objects.
So Gindlerod-Composed Jacob is working along side his Ghost counterpart. Gindlerod Jacob built the fire while Ghost Jacob watched. Once the fire was burning, Gindlerod Jacob went to Hurley and got the ashes, then brought them back to the fire and threw them in. Once the ashes were thrown in, which by the way were the positively charged counterpart to the matter in Ghost Jacob, Jacob was able to be seen by all. Thus the Gindlerod Jacob and the Ghost Jacob Fused and became the Daywalker!
Are these the kind of answers we are looking for with Lost? Long, complicated, drawn out, nonsense ramblings of the motives behind the writers' vision? I certainly hope not, because a show like this would suck. I am so happy the writers don't sit the characters in front of us and explain why Boy Jacob is there sometimes, and Adult Jacob is there sometimes. I am not a child. One can interact with objects, and one can't. That's all that is really important.