Secret Keeper/Playing With

Basic Trope: A character agrees to keep another character's secret.
 * Straight: Alice discovers that her best friend, Kate, is a werewolf and agrees to keep it.
 * Exaggerated: Alice is suspicious that Kate is hiding something from her so she follows her to discover her secret: she's a werewolf. Kate sees her and asks Alice if she can keep this from anybody which she agrees.
 * Justified: Alice expect to call favors from Kate in the future.
 * Alice is a werewolf (or a ghoul, or a mage, or a changeling) too, and plans to tell Kate sometimes in the future... after Kate feels comfortable having her around.
 * Inverted: Alice sells Kate's secret to the media.
 * An elaborate Masquerade is in place to make sure Alice, the only person who doesn't know Kate is a werewolf, never finds out.
 * Subverted: Secret Secret Keeper
 * Alternately, Alice tells Bob that Kate is a werewolf.
 * Double Subverted: ...but Kate eventually learns that Alice knows her secret and then asks her to keep it.
 * ...but then asks Bob to keep the secret, as well.
 * Deconstructed: Alice is having a nervous breakdown over trying not to spill the beans. She becomes increasingly paranoid, misinterpreting everything she hears as a reference to Kate's furry little secret.
 * Averted: Alice never finds out that Kate is a werewolf.
 * Or everybody knows that werewolves exist, and Kate happens to be one of them.
 * Lampshaded: ?
 * Invoked: Kate chooses Alice as a friend just so that she can keep her werewolfism a secret.
 * Defied: ?
 * Discused: "Don't worry, your secret's safe with me."
 * Conversed: "Yeah, her secret's safe alright...but for how long??"