I got an Eevee for free from the scientist girl (Amanita) in BW2, Castelia City after defeating the Elite Four. I evolved my Eevee into Glaceon. When I sent it out into battle, Glaceon had some kind of contest effect. The effect was a black screen and a huge star, followed by tiny stars shooting out. Glaceon has it's hidden ability, Ice Body, but many other Pokemon have Ice Body, except they don't have the effects.
That is the effect of using a Pokemon in a pokestar studios movie if you complete the movie with that Pokemon it will be considered a star there isn't anything special it does besides like a special entrance