lanciato l'incantesimo comando, l'avversario esegue l'ordine appena lanciato l'incantesimo o nel suo turno???

E' ben specificato...

Command

[...]

You give the subject a single command, which it obeys to the best of its ability at its earliest opportunity. You may select from the following options.

Approach

On its turn, the subject moves toward you as quickly and directly as possible for 1 round. The creature may do nothing but move during its turn, and it provokes attacks of opportunity for this movement as normal.

Drop

On its turn, the subject drops whatever it is holding. It can’t pick up any dropped item until its next turn.

Fall

On its turn, the subject falls to the ground and remains prone for 1 round. It may act normally while prone but takes any appropriate penalties.

Flee

On its turn, the subject moves away from you as quickly as possible for 1 round. It may do nothing but move during its turn, and it provokes attacks of opportunity for this movement as normal.

Halt

The subject stands in place for 1 round. It may not take any actions but is not considered helpless.

If the subject can’t carry out your command on its next turn, the spell automatically fails.