One of the first, if not the first, films to have characters talk over the lines of other characters, for a more realistic sound. Prior to this, movie characters completed their lines before the next lines were started.
The famous in-joke about Ralph Bellamy's character ("He looks like that actor...Ralph Bellamy!") was almost left on the cutting room floor: Harry Cohn, the studio head, saw the dailies and responded in fury at the impertinence, but he let Howard Hawks leave it in, and it has always been one of the biggest laughs in the film.
In the play the film was based on ("The Front Page"), the part of Hildy was played by a man. When director Howard Hawks was planning to make the film, he was going to cast a man. While auditioning actors, a secretary would read the lines belonging to Hildy. Hawks loved the words coming from a woman so much, they decided to rewrite the part for a woman.
Rosalind Russell thought, while shooting, that she didn't have as many good lines as Cary Grant had, so she hired an advertisement writer through her brother-in-law and had him write more clever lines for the dialog. Since Howard Hawks allowed for spontaneity and ad-libbing, he, and many of the cast and crew didn't notice it, but Grant knew she was up to something, leading him to greet her every morning: "What have you got today?"
The restaurant scene was written directly for the movie and took twice the time to shoot than expected: four days. The difficulty resided in the editing, since the characters had to eat, and the background actors kept walking around.
The play that this movie was based on ("The Front Page") had a famous last line: "The son-of-a-bitch stole my watch!" While the line and the plot points leading up to it didn't fit into "His Girl Friday", they did pay homage to it by having the first crime that Burns framed Baldwin for be the theft of a watch.
During the 1930s, Howard Hawks was hosting a dinner party when the topic of dialogue was brought up. He pulled out a copy of "The Front Page" to demonstrate the snappy exchanges between characters, taking the role of Burns. A female guest took the role of Hildy. While reading, Hawks realized the dialogue sounded much better with a female reading, and quickly secured the rights for the film from Howard Hughes. Ben Hecht (the author of "The Front Page") approved the gender change and the screenplay was put into production.
Ginger Rogers wrote that she was offered the role of Hildy Johnson. She read the script, but this was before Cary Grant was cast, and she turned it down. After learning that Grant was cast, she regretted it.
(at around 35 mins) In the news room the reporters are playing poker and one of them calls out to the reporter McCue, played by Roscoe Karns, who's sitting by a window that looks on a staircase. "Hey Mac (no response)... Hey Stairway Sam, would you mind turning on some lights." If you back up the DVD a minute or two you'll see why he's "Stairway Sam". In the background of these newsroom scenes McCue (or "Stairway Sam") is trying to look up the womens' dresses as they go up the stair case (at 27:30, 35:20 and 39:00). Even during one of the few serious moments in the film just after Molly Molloy confronts the newsmen with their lies Stairway Sam can't help himself. Not to mention he's flirting with and chatting up every woman who walks by. This was probably something that got by the Hayes office censors of the time.
Right after Williams is found in the desk, the Mayor tells Walter that he's "Whistling in the dark. Well that isn't going to help you this time. You're through." Walter says "Listen the last man that said that to me was Archie Leach just a week before he cut his throat." Archie Leach was Cary Grant's birth name.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
The calendar in the press room is from November 1938 which was approximately a year before the movie was made. It shows special days on the 8th, 12th and 24th which correspond to Election Day, Veterans' Day and Thanksgiving.
The trivia item below may give away important plot points.
Walter and Hildy hiding escaped killer Earl Williams in a desk in the city room was based on a real incident. Emile Gauvreau, the editor of the old New York City paper "The New York Evening Graphic", hid an escaped killer in the city room of the newspaper, interviewed him, wrote the story and waited until the paper was on the street before turning him over to the police.