In Book 9 of The Iliad, Achilles is still sulking in his tent, fuming over the disrespect shown to him by Agamemnon. Agamemnon had handed Achilles' sex slave back to her father to appease the wrath of the gods, and Achilles, thinking only of himself as usual, was not best pleased. Since then, the Achaeans have tried everything they can think of to coax Achilles out of his tent, but all to no avail.
The war's not going very well for them, and they need their best warrior out there in the thick of battle. But Achilles doesn't budge an inch. Not even a generous bribe of treasure is enough to move him. It seems for all the world as if the Achaeans are on the brink of giving up and going home. Diomedes may say that the Greeks can do without him, but deep down, everyone knows that just isn't true.
By the time we reach Book 24, Achilles has not just returned to battle—to avenge the death of his bosom pal, Patroclus—he's also changed the whole course of the war with his crucial intervention. Killing Hector has severely weakened the Trojans, who are in turmoil without their leader. When Hector's father, King Priam, approaches the Achaean camp to retrieve his son's broken corpse, Achilles puts aside his legendary stubbornness and agrees to Priam's heartfelt request. He can see shades of his father Peleus in Priam. Not only that, but he can empathize with Priam's grief as it so clearly matches his own over Patroclus.
If the theme of Book 9 was stubbornness and its dangers, the theme in Book 24 is the importance of empathy. Hector's death has clearly affected his loved ones deeply, and they lament his loss with great intensity. As for Achilles, his character appears to have undergone a considerable change from Book 9. He may still be a proud, fearsome warrior, but for the first time he seems to have an inkling of the damage that war can inflict on people. Through the death of his closest friend in battle and the heartfelt entreaties of Priam, he's developed empathy, which allows the Trojans at long last to give noble Hector, breaker of horses, a decent, dignified burial.