This is frequently interpreted as meaning ‘..the hard work is to be found in the detail..’ but I have another perhaps more ‘devilish’ explanation for it, than that.
In the pursuit of attaining that ‘detail’, a certain character trait is allowed to emerge. You become single minded or myopic and forsake all others. You become irrational, short of temper and are prepared to even cheat if needs be, all in the name of ‘detail’. In fact YOU become a downright little Devil yourself…
So as opposed to thinking the Devil Is In The Detail, it should read The Devil Is In You When Pursuing The Detail.