Trivia: When Rey has her vision after touching Luke's lightsaber, she hears a voice at the end that says, "Rey, these are your first steps." The voice that says "Rey" was a partial sound bite taken from the late Sir Alec Guinness saying "afraid," and Ewan McGregor came in to record "these are your first steps." So you're hearing both the voice of old Ben Kenobi and young Obi Wan Kenobi speaking.
Trivia: General Hux never blinks throughout the movie.
Trivia: Harriet Walter, who appears as a Resistance doctor treating injuries Chewie suffers, is the niece of Christopher Lee, who played Count Dooku in Attack of the Clones and Revenge of the Sith.
Trivia: A model of the tumbler from The Dark Knight trilogy is spray painted on the white hull of the Millennium Falcon, and mounted to its underside. This is because there was a friendly competition between JJ Abrams and Zack Snyder to see who could one-up the other by putting Batman/Superman references in Star Wars and vice versa.
Trivia: This is the first Star Wars film in which Harrison Ford receives top billing.
Trivia: Carrie Fisher's daughter has a small part - she is the young woman at the rebel base dressed in light brown, and her hair is styled in the famous Princess Leia buns. She has one line and is seen in the background quite a bit.
Trivia: James Bond actor Daniel Craig has an uncredited cameo playing the stormtrooper that releases Rey from the chair. He was filming Spectre on a nearby soundstage and was offered the chance to appear in the movie.
Trivia: This is the first "Star Wars" movie in which Kenny Baker is not cast as R2-D2. Instead, he is credited as "R2-D2 Consultant."
Trivia: Unkar Plutt, who Rey sells her haul to, is played by Simon Pegg.
Trivia: In Rey's force vision, before she sees R2-D2 and Luke's robot hand, she runs down a hallway - the same one from The Empire Strikes Back during the final lightsaber duel between Luke and Darth Vader. This sequence was originally going to be longer, with her seeing the duel itself from a new perspective (one of the reasons Darth Vader is one of the Force Awakens toys).
Trivia: John Williams did not want to compose the music for the bar scene because he thought it would be too derivative of the original. So the music in the bar is composed by an uncredited Lin-Manuel Miranda, creator of the musical Hamilton.
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.