This film was entirely improvised. There was no script, and the only things the actors and the crew had was a vague outline of the plot and the order in which certain events would take place.
All of the actors chose their character's names.
Kate and Luke reminiscing about Kate getting boiling wort in her eye was inspired by a real event. During pre-production, Jake Johnson and Olivia Wilde toured a brewery where they saw how beer is made. During the tour, a splash of wort got into Wilde's eye.
Anna Kendrick actually is drunk in the drinking game scene that takes place in the cabin. This scene was the last to be filmed that day, so Kendrick and Jake Johnson decided to use real alcohol instead of the prop alcohol provided. Because the movie is entirely improvised, this gave the actors more control over the scenes; in this case, Johnson's character is in charge of the game, so Johnson decided to make Kendrick drink more.
Director Joe Swanberg liked filming multiple takes, but he was so confident in the picnic scene between Ron Livingston and Anna Kendrick in the forest that he shot only one take. He told them that they wouldn't film more because he "didn't want to mess with it".