@haoxin I'll explain by example as I understood it. Say an enemy cats a spell and it takes 5s for the incantation (starting the casting until the spell actually manifests), and you use interruption, the spell fizzles.
If you use an attack with delay, the incantation now e.g. takes 7s instead of 5s. if you attack again, it takes (in total) 9s. In addition to the casting taking longer, the enemy can't move, so basically you can lock the enemies movement. if you now can apply more delay than time passes (e.g. you can add a 5s delay every 3s), you effectively paralyzed that enemy.
hth
Delay just causes an incantation to take longer. Interruption stops the incantation. Delay could be used to effectively infinitely delay a boss or other dangerous enemy from casting. But it requires everyone to be geared for it. Interruption stops the incantation, but allows the enemy to move. In the specific circumstances that a spell casting enemy is alone, delay would be better if everyone had it. But since most battles involve multiple enemies, just getting a hit to interrupt a spell will stop the enemy from casting fireball.
Kinda amusing how long they decided to dwell on that explanation. Basically Incantation Interruption is almost always a better option, but it isn't *strictly* better than delay.
His 'visible' job (i.e. the one that shows up on the card thing that popped out when they were doing the slave sale) is Explorer, and Sherry saw that at said sale, so her presumption is that he's an Explorer, hence asking for his level.
Roxanne and Sherry are also supposedly Explorers, so there'd be no particular reaction from him commenting on their levels (you normally can't job-change without special assistance, which they haven't done)