Can I use reaction spells like shield or counterspell when I'm in the middle of casting a spell with a long casting time and don't stop casting it?
The Sage Advice Compendium says:
Can you cast a reaction spell on your turn? You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius also has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.
Counterspell has only Somatic components. What about casting shield? That has Verbal and Somatic components. Do you have to stop casting a spell with a longer casting time in order to cast a reaction spell that has a Verbal component?