One of the film's nods to The Rockford Files (1974) is the yellow pages ad for Holland March's detective agency. It is almost a complete copy of the ad used for Jim Rockford. Like Jim Rockford, March keeps his gun in a cookie jar.
Similarly to the studio's marketing campaign for Inherent Vice (2014), Warner Bros. released a 70s-style trailer, complete with Betamax artifacts, an optical soundtrack audio crackle, and an "introducing" credit for Kim Basinger.
The signage at the fuel station showing EVEN refers to the 1979 rationing of fuel to the vehicles with numberplates that end in even or odd number on alternating even or odd-numbered days. For instance, if today is 3 April and your numberplate has ABC-827, you can fill your car on that day. Some states allowed both even and odd numbers on 31st of month.
At the beginning, when Jackson Healy is learning a new word (equanimity) while feeding his fish and reciting it to himself to memorize it, this scene is exactly like a scene from the Asian-American film Better Luck Tomorrow (2002); even the word is the same.
The dialogue between the bartender and the detectives ("He'll stop doing it." "Doing what?") is the same line from Iron Man 3 when Rhodey and Tony interrogate Mandarin. Both films are directed and co-written by Shane Black.
At the end of the film a marquee for the Comedy Store is shown in the background, featuring Tim Allen. "Allen started his career as a comedian in 1975. On a dare from one of his friends, he participated in a comedy night at Mark Ridley's Comedy Castle in Royal Oak, a suburb of Detroit. While in Detroit he began to get recognition appearing in local television commercials and appearing on cable comedy shows such as Gary Thison's Some Semblance of Sanity. He moved to Los Angeles and became a regular performer at The Comedy Store." [Wikipedia]
When Healey is fighting the Older Guy at Sid's house party, Healey delivers a punch combination exactly like the combination Maximus uses on Commodus in Gladiator (2000). Both Healey and Maximus are played by Russell Crowe.
When promoting his program The Loudest Voice on the Howard Stern show, Russell Crowe said this was one of two films where he wishes there was a sequel as he liked playing the character. The other film was L.A. Confidential.
The trivia items below may give away important plot points.
The take with Ryan Gosling sitting against the tree next to the body of Sid Shattuck is an homage to Lou Costello, who specialized in long takes where something, such as the Mummy or Frankenstein's monster, scared him so badly he couldn't make a sound, but eventually managed to yell his partner's name.
When Jackson, Holland, and Holly are in a cab surrounded by traffic after the big shoot out against John Boy at Holland's house, the black van that hit "Blue Face" can be seen next to the cab also stuck in the traffic.
Immediately after the bathroom stall scene, look closely at March's magazine, especially after Holly asks Healy to beat up her friend Janet: on the cover is Richard Nixon, who makes a cameo of sorts later on in the film.
If you look closely when Healy is leaving his apartment to deliver the "message" to March, before he shuts off the power in his apartment you can hear his TV mention the character John Boy from The Waltons (1971) playing on his TV in the bottom right corner of the screen. The hit man, John Boy, shares his name with the TV character, as well as featuring the same signature mole as the character.