In The Hobbit: An Unexpected Journey movie, Azog features in many places whereas I do not recollect him being mentioned in the book apart from the fact that he was the one who killed Thror, Thorin's grandfather.
How does Azog feature in other Middle-earth canon? Who did he show allegiance to and what were his motives?
As per the Tolkien Gateway Azog entry, he appears in the following places in canon:
In the canon Azog is dead. He is killed by Dain in the battle at Moria. Azog's son Bolg is who leads the Gobins at the Battle of Five Armies.
The Lord of the Rings
Azog first appeared as a named character in the appendices to The Lord of the Rings, published in 1955. There is a section in Appendix A which recounts some of the history of the dwarves, about 45 years prior to the start of The Hobbit.
Thror and Nár journey to Moria. Thror ignores Nár's advice and goes inside. Nár waits around in hiding for a few days and sees Thrór's body throw on to the steps.
Then Nár came up, and found that it was indeed the body of Thrór, but the head was severed and lay face downwards. As he knelt there, he heard orc-laughter in the shadows, and the voice said:
‘If beggars will not wait at the door, but sneak in to try thieving, that is what we do to them. If any of your people poke their foul beards in here again, they will fare the same. Go and tell them so! But if his family wish to know who is now king here, the name is written on his face. I wrote it! I killed him! I am the master!’
Then Nár turned the head and saw branded on the brow in dwarf-runes so that he could read it the name AZOG. That name was branded in his heart and in the hearts of all the Dwarves afterwards.
The Lord of the Rings - Appendix A Annals of Kings and Rulers, III Durin's Folk
There then follows The War of the Dwarves and the Orcs, which the dwarves spent three years preparing for, and then when ready "assailed and sacked one by one all the strongholds of the Orcs that they could find from Gundabad to the Gladden" in their hunt for revenge against Azog. (Incidentally this happens to be the war in which Thorin earns his "oakenshield" name.)
At the last battle before the gates of Moria, Náin challenges Azog to come forth
Then Na´in stood before the Gate and cried with a great voice: ‘Azog! If you are in come out! Or is the play in the valley too rough?’
Thereupon Azog came forth, and he was a great Orc with a huge iron-clad head, and yet agile and strong. With him came many like him, the fighters of his guard, and as they engaged Na´in’s company he turned to Náin, and said:
‘What? Yet another beggar at my doors? Must I brand you too?’ With that he rushed at Náin and they fought. But Náin was half blind with rage, and also very weary with battle, whereas Azog was fresh and fell and full of guile.
ibid
Azog slays Náin, but then notices that the battle isn't going well and turns to flee, wherapun he's killed by Náin's son Dáin.
Then Azog laughed, and he lifted up his head to let forth a great yell of triumph; but the cry died in his throat. For he saw that all his host in the valley was in a rout, and the Dwarves went this way and that slaying as they would, and those that could escape from them were flying south, shrieking as they ran. And hard by all the soldiers of his guard lay dead. He turned and fled back towards the Gate.
Up the steps after him leaped a Dwarf with a red axe. It was Dáin Ironfoot, Náin’s son. Right before the doors he caught Azog, and there he slew him, and hewed off his head. That was held a great feat, for Dáin was then only a stripling in the reckoning of the Dwarves.
ibid
The Hobbit
In 1966, Tolkien revised The Hobbit for its third edition. One of the changes that he made was to back-insert in the character of Azog into the roll of the previously unnamed father of one of the antagonists.
In the 1st and 2nd editions, it was said that Thrór was killed by "a goblin".
“I did not ‘get hold of it,’ I was given it,” said the wizard. “Your grandfather was killed, you remember, in the mines of Moria by a Goblin — ”
“Curse the goblin, yes,” said Thorin.
The Hobbit (1st edition, 1937) - Chapter 1 "An Unexpected Party"
This was changed in the third edition to Azog:
“I did not ‘get hold of it,’ I was given it,” said the wizard. “Your grandfather Thror was killed, you remember, in the mines of Moria by Azog the Goblin.”
“Curse his name, yes,” said Thorin.
The Hobbit (3rd edition, 1966) - Chapter 1 "An Unexpected Party"
At the same time a footnote was added to Bolg's appearance, to specify that his unnamed father was Azog.
“Halt!” he called in a voice like thunder, and his staff blazed forth with a flash like the lightning. “Dread has come upon you all! Alas! it has come more swiftly than I guessed. The Goblins are upon you! Bolg* of the North is coming, O Dain! whose father you slew in Moria. Behold! the bats are above his army like a sea of locusts. They ride upon wolves and Wargs are in their train!”
* Son of Azog. See p. X
The Hobbit (3rd edition, 1966) - Chapter 17 "The Clouds Burst"
From Appendices we can reconstruct this character. First fact: he is an orc chieftain. Such guys are always more capable fighters, stronger, faster, more agile, the elite of the orc race. Their ,,subjects" are fiercely loyal to them (wanting to avenge their deaths). There is not much details about his life, but at one point he took his people to Moria and which is surprising he deemed himself to be an independent king there! That's an ambition :), anyway he wasn't specifically keen on hunting down the line of Durin as movie states. It just happened that Thrór came there with one companion Nar acting proudly ,,as it's rightful heir returning". Azog shows some of his skill and traits in the fact he is literate, he is proud and knows value of money :) he spares Nar and mockingly throws pouch of small money as ,,payment" for him sending message to family of Thrór. This act of murder, desecrating corpse as a general insult to dwarven race initiated the War of the Dwarves and Orcs, so Azog must have been the leader of all orcish population in Misty Mountains with Moria as his seat of power! This implies he was very powerful among his kind.
In The Hobbit he serves as a justification of orc's pursuits and special hatred towards dwarves. His son Bolg wants to avenge him and additionally ,,gain dominion over whole North" other orc communities want to avenge the death of Great Goblin and of their comrades, also to claim the Erebor's tresure for their own. Bolg rallies the orcish tribes at his capital Mount Gundabad to fulfill these goals, his parentage was the reason he so easily assumed control over orcs, he inherited the role of a leader.
