Anna Kendrick is actually drunk in the drinking game scene that takes place in the cabin. This particular scene was the last thing that needed 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.
The piece of dialogue between Kate and Luke where they reminisce about Kate getting boiling wort in her eye, was inspired by real events. During pre-production, Jake Johnson and Olivia Wilde toured a brewery where they saw how beer was made. During the tour, a splash of wort got into Wilde's eye.
The director liked filming multiple takes, but was so confident in the picnic scene between Ron Livingston and Anna Kendrick in the forest, that he only shot one take. He told them that they wouldn't film more because he "didn't want to mess with it."
Director Joe Swanberg started shooting this film at the same time his friend David Lowery was shooting Ain't Them Bodies Saints (2013). They decided that they, '...should have the same character in both movies just as a little hat tip. There's a cop named Gene Dentler in his movie and '[Jason] Sudeikis' plays Gene Dentler in ours. The name plate on Sudeikis' desk (in Drinking Buddies) that says Gene Dentler, we sent down to Shreveport and [Lowery] shot it in his movie, too."
The trivia items below may give away important plot points.
Ron Livingston kissing Anna Kendrick's forehead after they kiss wasn't supposed to be in the scene. The director told the actors that the scene would end after they kissed, but Livingston felt so guilty and sad when he saw Kendrick's facial expression that he instinctively kissed her forehead.
Director Joe Swanberg had Chris (Ron Livingston ) not appear in the last third of the movie, but had the characters refer to him in order to make audiences wonder whether he and Jill (Anna Kendrick) started an affair.
Because there was no script and the making of the film was so up in the air, the actors would show up on set often not knowing what they would be filming that day. Ron Livingston arrived in Chicago (where the movie was filmed) one day before he was scheduled to work, but then received a call telling him that filming was ahead of schedule so he would be working that day and that he would be filming "the break up scene". Livingston knew that his character had a relationship with Olivia Wilde's character and to a certain extent with Anna Kendrick's too, so when he arrived on set, he had to ask the director which one of the two he was breaking up with and why.