Even though Fortinbras is a character who remains unseen for most of the play and only enters after Hamlet's death in the very final scene, it is clear that Shakespeare draws many comparisons between these two characters, and clearly indicating the way in which Fortinbras acts as a foil for Hamlet in the same way that Laertes acts as another foil.
Thirty years earlier Hamlet's father (old King Hamlet) had slain old King Fortinbras in a fatal duel, winning a piece of ground to which young Hamlet fell heir. Young Fortinbras therefore must be at least 30 years old. But while old King Hamlet lived, young Fortinbras never dared to attempt vengeance against old King Hamlet nor to reclaim those lands lost by his father, with all bonds of law. But the cowardly young Fortinbras believes that he has a better chance of reclaiming the land from Claudius. For one thing, Claudius won't challenge him to a personal duel. The two "brave" princes (Prince Fortinbras and King Claudius) can fight it out with no royal bloodshed - just a few thousand underlings.
Hamlet was possessed by his father's ghost - at least metaphorically and psychologically, if not supernaturally. This means that for much of the play Hamlet is expressing his father's opinions rather than his own. Some of his soliloquies are actually debates between Hamlet himself (before he was "from himself taken away") and his father's spirit which now lives all alone in his brain.
This would be very challenging for an actor to convey. A specific example: the "my thoughts be bloody" soliloquy. Hamlet starts the speech as himself, the scholar from Wittenberg: ...he that made us with such large discourse, Looking before and after, gave us not That capability and god-like reason To fust in us unused. But then he abruptly comes under the sway of his father's anti-intellectual, warlike spirit: some craven scruple Of thinking too precisely on the event, Culminating in "my thoughts be bloody or be nothing worth!"
