On the behind the scenes special feature included with the film, Maisie Williams reveals that no matter what outfit Skye is wearing, she has on her tights and boot-style Converse All Stars because Maisie herself chose them as something that she thought fit the character very well.
Maisie Williams is primarily known for playing Arya Stark on Game of Thrones. Interestingly, both Arya and Skye have a list of tasks which are important to them. Skye has a list of things she wants to do before she dies, and Arya has a list of all the people she wants to kill.
Asa also play on movie called Time Freak where he played as a teenage physic genius who invents a time machine to go back in time and fix his relationship with his girlfriend, played by Sophie Turner-who play as Sansa Stark, Maisie's co-star on Game of Thrones.
The trivia items below may give away important plot points.
When Calvin receives the bunch of cards in the mail, the one he opens says "Calvin, Wow! 9 years old! What a strange kid you probably were. But if we had been together I'm sure we would have been best friends or at least sworn enemies. You're (sic) 9th birthday was probably the worst one. You probably missed your sister. And probably no one told you how lovely and special you are. So I'll tell you now. You are lovely and special. Love, Skye"
When Maisie Williams and Asa Butterfield meet for the first time on the support group and they were asked to made a list about what they want to do, Maisie's character Skye said "I won't take you to Amsterdam" this is a nod to the Fault in Our Stars movie where Gus (Ansel Elgort) take Hazel (Shailene Woodley) to Amsterdam with his 'cancer wish'