Mollie Malloy (Helen Mack)
In His Girl Friday, we don't learn what Mollie's profession is. But, in the original theatrical production of The Front Page, Mollie's a prostitute. That helps explain why the newspaper reporters treat her so badly, and perhaps why she's desperate enough to throw herself from a window.
She's in a stigmatized, precarious profession, and the having her name bandied about in the paper could get her arrested, destroy her relationship with her family, and generally ruin her life.
In 1940, you didn't get to talk about prostitutes on film, though, so the character of Mollie doesn't end up making a lot of sense. It's not clear why the newspaper reporters treat her so badly, or what she has to lose. The character seems to mostly exist to yell at the newspapermen and tell them how awful they are.
Mrs. Baldwin (Alma Kruger)
Bruce's mom, Mrs. Baldwin, is an angry, very proper Mommy Dearest. It's not a new part, but to her credit actress Alma Kruger does look incredibly indignant when, as Mrs. Baldwin, she's picked up and hoisted away by Walter's henchman.
Duffy (Frank Orth)
Walter's copyeditor. He's mostly just a sounding board so Walter has someone to talk to while he talks and talks and talks. Duffy doesn't get enough words in edgewise to have much of a character.
Dr. Egelhoffer (Edwin Maxwell)
The film's main message is, "don't trust politicians." A secondary message is "don't trust newspapermen, except when they're telling you not to trust politicians." A lesser message is "don't trust psychologists"—or at least don't trust one if his name is "Egelhoffer."
Evangeline (Marion Martin)
Evangeline hardly has any lines; mostly she just stands in the background looking seductive and amoral. Like Louie, she shows Walter's unscrupulousness—though the film can't quite say how unscrupulous because it's not quite ready to acknowledge that prostitutes exist.
Sheriff Peter B. Hartwell (Gene Lockhart) and the Mayor (Clarence Kolb)
Hartwell is incompetent and corrupt, and the Mayor is corrupt and incompetent. Either way you arrange it though, they're both politicians, and that makes them the bad guys when your heroes are the crusading pressmen.
Louie (Abner Biberman)
As Walter's criminal associate, Louie works primarily in the story to highlight Walter's awesomeness. In the first place, Louie's uncouth, thuggish and not too bright (a stereotype of a "dumb immigrant" as Walter says—what a charmer that Walter is!). His subservience and general confusion (he thinks "albino" is a nationality) contrasts with Walter's perfect mastery and cool.
At the same time, Louie shows just how dangerous and smoothly ruthless Walter is. After all, Walter has his own gangster on the payroll.
The rough-and-tumble, cynical, pressmen rag on Hildy and the Mayor and anyone else they can manage to rib. But still, there's a fundamental decency peaking out every so often from beneath their shaky morals.
The film thinks the press is a bunch of slimy invertebrates who would lie about their own grandmother if it would make a story more scandalous. ("His grandfather was a snake" Hildy says of that pressman among pressman Walter.) But at the same time, those slimy invertebrates are our slimy invertebrates, protecting democracy by keeping tabs on the even slimier politicians. You need slime to fight slime.
And if the press is just a bit wittier and cleverer on film than it could ever be in real life? Well, that's Hollywood.
Joe Pettibone (Billy Gilbert)
The portly Pettibone, who delivers the pardon and continually touts the wisdom of his wife, is played by notable comedian Billy Gilbert. Gilbert appeared with everyone from Charlie Chaplin to the Three Stooges, but is probably best known for a role he didn't play. His famous comic sneeze routine was the basis for the Sneezy in Disney's Snow White and the Seven Dwarves.