Solas gets really nervous in tightly enclosed spaces like caves. Considering he likes to roam and the freedom of knowing he can leave, being in a place with little room to maneuver and unknown ways out, it’s safe to say he’ll be visibly uncomfortable. Old ruins can sometimes be very claustrophobic, but there’s history there, and Solas can often escape to dreams once it becomes overwhelming. That, and old ruins spark his curiosity, and he knows there’s always a way out.
As long as he knows the situation is controllable and in his hands, he’ll be okay. But if they cannot avoid the caves, he’ll probably remain unusually silent and even appear to be antsy. Taking him on the Descent DLC would probably be a bad idea, hence why he’s so unusually quiet and only makes a comment from time to time (is nemo explaining why there wasn’t much dialogue then? Yes).
How about Zen getting a parrot who can mimic, 707 is going to ruin this, and sing? (Maybe the bird looks terrible at first but after prolonged affection and bonding, looks beautiful? Kind of like child Zen?)