Fairy/imp/leprechaun doesn't really give me a flavor for the critters personality, I don't know if you've got a Tinkerbell or Elmo like character who's just suddenly giving a stern lecture or if your nominal fairy has overtones of Sidhe, Redcap, Bannik, etc. In any event, that character's personality is going to dominate how that goes and I don't know what that is. If a chewing out 'Grow a spine' lecture is specifically coming as a surprise form the character, who is otherwise friendly or whimsical, then you're probably better off using stock phrases, or quotes, like the drill instructor from Full Metal Jacket or something. The first thing that came to mind, what with fairies often granting wishes, was an old quote, by Paddleford I think, "Never grow a wishbone where your backbone ought to be."

Anyway, that's about all I can suggest, I really have no idea about the character, or even if you're aiming for cheezy or serious and if the latter whether your Fairy critter is more Walt Disney, Brothers Grimm, or Jim Butcher.