I'm going to post these tidbits occasionally in case people haven't seen all of the spontaneous quest dialogs available. It's been rather frustrating gathering all of these. There are some quests where Dorian has no comment at all (tested by bringing only him). Sometimes the game picks randomly among all three followers. I've also learned not to take Solas along anytime there are magical remarks because he has a tendency to override Dorian, such as in the Solasan temple. On occasion, Dorian's remark will be dependent on that of another follower, since he responds to them.
During Solas's quest Measuring the Veil...
Inquisitor: What manner of fire is this?
Dorian: I've heard of this! It's veilfire. Tevinter mages discovered it.
On seeing a rune for the first time...
Dorian: What are those runes? I can't make them out.
On lighting the torch for the first time...
Dorian: Marvelous. This is fascinating.
[The tone of this is supremely adorable.]
On revealing a rune for the first time...
Dorian: The light from the torch is affecting the runes.
[To get this I had to extinguish the torch I got from lighting it the first time -- via Dorian -- and run back to get it again.]
On reading the rune...
Dorian: A Tevinter weapon enchantment. Oh, this could be very useful.
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I find these remarks interesting in that it's one of the few times where he is being enthusiastic, rather than cynical, about Tevinter. He may think and know that his homeland has many virtues, but most of the time he is rather negative about it, and with good reason.