Why did the diary Horcrux create a fully fledged Tom Riddle who could walk and talk and stuff when no other Horcrux seemed to do that?
What was special about the diary that allowed it to do this?
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Not this question again! See my answer at scifi.stackexchange.com/a/40805/4918 "If diary Tom Riddle had succeeded, would there have been two Voldemorts?" , scifi.stackexchange.com/a/14636/4918 "How come Nagini had magical powers". It's not true that no other Horcrux did that, because the locket and the snake did pretty convincing illusions too, so it's at least three out of seven. – b_jonas Aug 23 '16 at 22:29
Why did the memory inside the diary acquire a quasi physical form?
The Diary was no ordinary diary. It had the power to draw life force from a reader/writer who got emotionally attached to it and use it to create its own body. As it happened, Ginny poured her heart out to Voldemort, strengthening him and making herself vulnerable to him.
So Voldemort used Ginny's life force to create a quasi-body for him. He would have succeeded in creating a full body, Had Harry not intervened.
Voldemort did not bestow the power to draw life force to strengthen the Horcrux inside any other of his relics except the diary which is why it stands out.
What impacts the diary's success in chamber of secrets could have had, JRK had following to say, (quoted from DVK's answer):
Q: In 'Chamber of Secrets', what would have happened if Ginny had died and Tom Riddle had escaped the diary?
A: I can’t answer that fully until all seven books are finished, but it would have strengthened the present-day Voldemort considerably.
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Given the question has been chopped in two, I've taken the liberty of editing your question to remove the now-irrelevant bit. – Valorum Aug 23 '16 at 19:24
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If he reposts the other half, there's nothing to stop you going into the edit history and copying it over. Or if he doesn't ask, posting the question yourself with a self-answer. – Valorum Aug 23 '16 at 20:01
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1@Aegon Second half reposted: scifi.stackexchange.com/questions/138660/… – Rand al'Thor♦ Aug 23 '16 at 21:19
The diary couldn't create a Tom Riddle body straight away. Like most Horcruxes it started out looking like a normal object. Ginny then began to pour her problems into the diary and it slowly took hold of her. At this point the Horcrux began to drain Ginny's strength and used it to create Tom Riddle.
Eventually the Horcrux would have enough strength to bring Riddle back to life entirely but was stopped before this could happen. When the diary was destroyed Ginny's strength appeared to return to her. Other Horcruxes didn't have someone dependent on them in the same way so couldn't drain their strength.
