Defensive casting mode allows casting without triggering an attack of opportunity. However a concentration skill check (DC 15 + spell level) must be made or else the spell is lost. There is a -4 penalty (increasing the DC) if there is an enemy within 4 meters and the caster does not have the combat casting feat. Thus, without that feat, it takes a concentration skill of 27 to guarantee passing this mode's check for any spell.
Defensive casting is a combat mode. It cannot be used at the same time as other combat modes. Movement and other non-combat actions will cancel the mode.