The studio tried to recreate the success of Twilight (2008) with this film by marketing it as a "Twilight for boys". Although it hoped to be a box office success, the movie did not earn the hundreds of millions that the Twilight films grossed.
Dianna Agron landed her role after a chance meeting with executive producer Steven Spielberg at a charity event. As a big fan of Glee (2009), Spielberg was seriously considering Agron for a prominent part when the chance encounter proved beneficial for both parties.
Just like the aliens are from Mogadore, which is their planet, but also a play on the small Ohio town of the same name, the planet which John and Number Six are from, Lorian (pronounced LORE-EE-un), is a play on the small Ohio town of Lorain (pronounced LORE-rain), which is about one hour northwest of Akron.
The evil aliens in the film are called "Mogadorians" - apparently they're from the planet Mogadore. Mogadore also happens to be a small suburb of Akron, Ohio - not far from where the film takes place, and not far from where James Frey, one of the writers of the novel on which this film was based, grew up. The word means "beautiful" in Arabic; it's also the name of a town in Morocco.
The trivia items below may give away important plot points.
Several of the ending plot points were setups for a potential sequel, including coming back to Sarah in Paradise, Sam being "one of us now," the search for Sam's real father, what's in the box, and more.
Right before he dies, Henri (Timothy Olyphant) tells Number Four "You have no idea what you are capable of". Timothy Olyphant uses almost the exact same line in Live Free or Die Hard (2007) when he tells John McClain "You have no idea what I am capable of".