Much of what we have previously been told about spirits came from either Andraste herself or filtered through the Chantry. However, Anders actually had a spirit co-existing with him, so presumably Justice must have been able to give him some insights.
Strangely enough Solas reminds me a lot of Justice/Anders. Anders on his own was a pretty regular sort of guy. He was a bit selfish and self absorbed but his principle specialty was as a spirit healer, which suggests he did care about others. Then Justice suggests that instead of just worrying about his own freedom, he should consider the fate of his fellow mages as well. So they merge and Anders becomes far more focussed and single minded. He still opens a clinic to help the poor of Kirkwall (which the old Anders probably would have done too) but it is the mage situation that really directs his attention, particularly after what happened to Karl. Thereafter he is working towards bringing the Chantry and the Circles down. That is rather like Solas with the evanuris. He seemed content not to take action against them, despite the fact that they were already false gods and slaves suffered under them, until they killed Mythal. That was what focussed his attention.
Then he became obsessed with the fact that raising the Veil didn't have the desired effect and he had freed the elves, only for them to be reduced in power and enslaved again. So now he wants to put things back the way they were, regardless of the affect it will have on so many innocent bystanders. Just as Justice/Anders blew up the Chantry in Kirkwall without thinking about the collateral damage or the implications for the other mages. In fact he says to Orsino "You were dead already", because living inside the Gallows under Meredith was no sort of existence to his mind, just as Solas didn't see the survivors of his actions as real people, but tranquil. It is very much the single minded obsession of a spirit. However, when both draw on their mortal persona, they are able to empathise with their victims; Anders was surprised if you spared him because he fully expected to die for what he had done and I think this may lie behind the whole redeem him or kill him option for Solas.
If you were rivalled with Anders, tried to find out what he was going to do before he set the bomb off and dissuade him from his plan, Justice comes to the fore and gets very aggressive with you. Clearly he didn't want you getting to the "human" Anders and persuading him otherwise. So it will be interesting to see if a similar sort of thing happens with Solas.
I was reminded by a poster at work yesterday of the old native Amercian story of the two wolves that war in your mind for dominance, one virtuous, humble and kind, the other arrogant, self pitying and destructive, and wondered if the writers drew on this idea for Solas. At the end of the parable the old storyteller says the wolf that wins is the one you feed. That could certainly apply to Solas.