Gandalf’s face is blackened and mucky. The wizard carries the dark of Moria into his pores. The face of the wizard seems part of the landscape of the mine. This enhances the sense of an ancient wisdom born of experience. Gandalf is part of this place. Wisdom is about to be imparted to the Hobbit.
Frodo’s face is clean and unsullied. This enhances the sense of innocence and inquisitiveness of the hobbit. The look is of a pupil visiting a wise headmaster. Gandalf is Frodo’s experienced mentor. Though the light is falling of Frodo’s face it is Gandalf’s face that seems to radiate perception and wisdom.
The heads of the two move in opposite directions. Frodo is drawn to look down at Gollum while Gandalf remains still. Gandalf rests in his higher knowledge about the creature Gollum and his part in the drama.

The pair finish the exchange looking the same way. This suggests that Gandalf and Frodo see the situation the same since Gandalf has imparted the wisdom

Throughout the scene Gandalf is in shadow and Frodo in the light. The suggestion is that Gandalf’s wisdom comes from an understanding of the forces of the shadow in middle earth. In shadow logic there is often a reversal. The creature Gollum may yet be an agent for the quest despite Frodo’s urge to seem him gone. Gandalf’s advice about fulfilling destiny is given to Frodo from the shadow. From the shadow of Frodo’s recollection of these words Frodo will have the courage to go on when the Fellowship is broken at Amon hen. Again Gandalf will speak as his mentor out of the shadow.