Is there any difference between ‘it’ and ‘so’ as a complement of ‘she looks’?

Question

D’you – d’you want to go to the ball with me?” said Harry. Why did he have to go red now? Why?

”Oh!” said Cho, and she went red too. “Oh Harry, I’m really sorry,” and she truly looked it. “I’ve already said I’ll go with someone else.” (Harry Potter 4 [US Version]: p.397)[Bold font is mine]

I can understand ‘she truly looked it’ means ‘she truly looked sorry’. But if I write this kind of sentence, I’ll probably write ‘she truly looked so, as I’m familiar with ‘I think so.’

Is there any difference if you write ‘she truly looked so’ instead of ‘she truly looked it’?

Answer 1

I think "she looked it" is more idiomatic.

It's equivalent to saying "She was the picture of sorrow"

Saying "she looked so" is correct, but a litle unusual.

Most readers would be left thinking "she looked so .., what?"

Answer 2

Both usages are valid in this instance, although there might be some out there who would find the usage of "so" at the end of this sentence to be awkward, expecting the sentence to continue.

For a more pronounced difference between "it" and "so" and how they are not always interchangeable, compare "I think so," and "I think it."

Answer 3

"So" is idiomatic replacing a "that" clause thus:

He said that he was happy => He said so.

Similarly "think", "hear", "guess", "suppose", "expect".

It is much less idiomatic for an "as if" clause

She looked as if she was sorry  =>?  She looked so.