When can I cast an at will spell/ability is it an action?

@mikemearls dnd question: when can i cast an at will spell/ability? Is it an action? Or a spell casting mechanic? @ChrisPerkinsDnD

— Uhtred (@RocketshipPilot) March 12, 2016

@mikemearls @ChrisPerkinsDnD in other words, is 'at will' instantaneous? Or does it mean not requiring a spell slot?

— Uhtred (@RocketshipPilot) March 12, 2016

@RocketshipPilot @ChrisPerkinsDnD means not requiring slot. Uses action as per spell.

— Mike Mearls (@mikemearls) March 12, 2016