Actually, that's part of what makes spell like abilities so frightening. From the SRD:<br/><span class="bbc_font" style="font-family: courier;">A spell-like ability has no verbal, somatic, or material component, nor does it require a focus or have an XP cost. The user activates it mentally. Armor never affects a spell-like ability’s use, even if the ability resembles an arcane spell with a somatic component.</span><br/><br/>Spell Like Abilities just require a little concentration. You can add in effects - motions, glowing eyes, auras of power & all sorts of cool things - but in truth, the monster just thinks for a few seconds & suddenly all manor of magic just happening. I suppose the AoO is based on the mental focus & concentration required to just 'pull' reality into order under your will & make magic happen.<br/><br/>It's really frightening to imagine - no clue, no sign. Just a moments pause & suddenly meteors are falling. Demons vanish. Devils burn & Angels heal the masses. All in mere moments. Although, it's much more fun when you add in some shiny effects :P