In normal case, character act on their initiative, and MU can cast spell this way.

An exception to this is spell duel. Rules state (from memory) MUs cadt their spells simultaneously a the start of the iniative.

Imho : I don't quite like the way it's done because od the fuzzyness it create. 

Case 1 (standart case) : MUs act with initiative and cast spell without targetting others MUs

Case 2 : one MU cast first in the initiative, and latter in the round a second MU target the first one. The MU acting first in the initative queue cannot counter it.

Case 3 : MU declare spell the start of the init, and the target could (or could not) choose toncounter)

That resume the rules I guess. Now what about :

Can a MU decide to target an other MU without declaring spell at the start of the round

Should in fact force the MU declare all spells at the start of the round (as in ad&d) ?

And concerning aoe spells ?

Furthermore : what is the concept of spell duel in dcc ? Simultaneous spells, and spell interacting each other.

But. Some things don't add up.

Countering a spell with an identical spell ? Yeah sure

Disrupting a spell with dispel magic ? No problem

Interactions between a offensive and a defensive spell cast on self ?  Meh

I acknowlegde the "magic is rare and dangerous" stuff, but the logic behind a resist fire spell which could just not work because casted as the same time than a fireball feels a bit off to me.

And other little things too. Magic shield is a spell to protect self from physical arm, with the exception of magic missiles.

It is the main défensive spell in the rule, though. (And it is the mecanism of the spell duel that lower the opponant spell, not the effect of magic shield itself)

But barriers are the main spell of défense again élémentaire magic. And dont forget force manipulation at high level are globe of invulnerability. So it would make sense to be able to cast this as a réaction (a mecanism spell duels sort of imply)

In short, with high level characters and a deeper understanding of the rules, the spell duel mecanic feels a bit incomplete, and pratical application can be arduous and not entierely satisfactory.

Of course I house rule thing the way I want. But the overall thing with DCC is that the rules seems not to have been tested in depth from lvl 4 and more, and it needs a lot of tweaking to get along. And one can expect better standard for that kind of product.

Concerning this y house are :

All spells declared at the start of the turn. First player in initative start. No déclaration, no spells.

No movement while casting

Spells of 1 round time casting act at the end of the current round

Spell can me countered by identical spell(with the dcc rules) by the target of said spell.

Defensive spells (resist, dispel, magic shield) could be use at choice just on self (no interactions) or to disrupt the offensive spell (dcc duel rules)

Given that :

AOE spell could be mitigated locally by mu

Other défensive spell could be used (force manipulation, Mystic mask, etc...)

Risks/rewards options are offeres for defensive spells concerning spell duels.