If the trigger to a readied action involves a creature moving into range of a spell or attack, is the creature considered to be fully within than range when the readied action is taken?
Example: Say you’ve got an inquisitor facing down a longspear-wielding Hill Giant. The inquisitor readies an action to cast Rebuke when the giant enters the spell's 20 ft. range. The giant, who has 20 ft. reach, moves into a space 20 feet away from the inquisitor. When the inquisitor casts the spell (non-defensively), can the giant take an attack of opportunity? Does it matter if the giant is making a regular move, or taking a 5-foot step?
Depends on exactly how you set your triggering criteria. If you wanted something to happen just before the giant's in reach (perhaps you're casting a pit spell), then word your trigger appropriately. If you want to hit him with a spell that has a 20 foot range, word your trigger so that you cast as soon as he's in range.
Range is reciprocal. If the giant has a reach of 20 feet and the spell has a range of 20 feet, there's no way that the giant can be in range of the spell but not within reach.