Hi Ossian, This forum is primarily for how to use Roll20 rather than how to play any specific game. I suggest heading over to the official D&D Discord server, they can answer your future questions there. With that said: 1) Shield takes effect before the attack is completed. As a result it can cause a miss in the case of an attack, or negate magic missiles. 2) Spellcasters are no longer interrupted while casting, generally you do not stop a spellcaster from casting a spell. In the current edition if a spellcaster takes damage while casting nothing happens UNLESS the spell has a casting time longer than 1 action (or reaction). In that case they have to concentrate on casting (see Concentration rules) and the damage might disrupt the spell, but unless the damage is pretty high it isn't likely to. The Con save DC for most hits (under 22pts of damage) is only a 10 and many spellcasters take steps to ensure that concentration checks are made with bonuses such as Advantage. Next, many spells now have "Concentration" added on, which means you can only have one of those spells active at one time. Concentrating on a spell can also be disrupted as above. The only ways to stop a Sorcerer casting in the middle of a battle is to incapacitate them, or use the spell Counterspell against them. Also: There is no more casting defensively, no more attacks of opportunity (now called Opportunity Attacks) against a spellcaster for simply casting a spell. For that matter, Opportunity Attacks (formerly Attacks of Opportunity) simply don't exist anymore for the majority of things. For the most part, only movement provokes one now.
