Virago said:Of course, an intelligent fighter could respond to that by readying their action to follow you and attack you when you cast.
The 5' step cannot be taken unless the fighter started out his turn adjacent to the spellcaster, which is not always (or even usually) the case.
It's actually rather hard to interrupt or prevent spellcasting in melee. Readying a melee attack is not usually the best way.
Caliban said:The fighter could do a Partial Charge with his readied action, if he doesn't move before readying the action.
Archer said:Of course defensive casting almost never fails for higher level wizards.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.