Surprise Guest interactions happen one of two ways: by random chance after a player completes a stage battle, or at certain times on the home screen when the speech bubble icon appears (and is tapped by the player).
If a Surprise Guest interaction happens post-battle, the character that appears will always be one of the three represented by the Demon Cards the player battled with. If a Surprise Guest interaction happens via the home screen, the character that the player has set on their home screen is the only surprise guest available for these interactions.
The purpose of Surprise Guest interactions is to raise a player's intimacy score with the appearing character, which can be done by tapping the character in certain places or by giving the character presents. Each character has particular tastes in regards to both tapping techniques and items, which is why separate guides have been created for each available character.
Surprise Guests can also give the player presents up to six times per day, but presents will only be given if the player achieved heart reactions (video). The presents given are always two each of: Raven x1, Grimm x1,000, and AP x10.