In dialogue, Eugene O'Neill revolutionized western theater with his dedication to using the American vernacular. This is the most characteristic aspect of his style. O'Neill was part of the movement of "realism" in theater, wherein the goal was to recreate reality as accurately as possible. O'Neill was one of the earliest American playwrights to extend this realist theater to the words that real people use. While the Tyrones speak clearly and in altogether standard English, their language is still particularly American.
This is observable in the vocabulary that the characters use, as O'Neill suffuses the Tyrones' speech with Americanisms like "bum," "a fake," "licked," and "lingo." Jamie and Edmund, in particular, always have snappy, smart retorts and use American slang. Even with the family's penchant for quoting Shakespeare, the playwright's words are often put next to especially coarse slang. When O'Neill was writing, this was a revolutionary level of realism in dialogue. Mary, though, is somewhat different: she tends to speak in clichés, idioms, and platitudes. O'Neill shows that most of Mary's thoughts are not under her control, and she tends to speak almost on autopilot, words forming thoughtlessly into sentences.
In his stage directions, O'Neill is also peculiar. Sometimes he describes full choreography for a moment or even a small scene in the stage directions. These are typically done, as may be expected, in terms of specific movements for the actors. However, these directions tend to be more expansive and particular than the average play. O'Neill, unlike many playwrights, takes great care with every last feature of the visual appearance of his story. His script leaves little interpretation to directors, either in movement or in casting, speaking to the specific look of the shine of an actor's eye, the curve of their arm, or the deepest manner of their character. O'Neill is also a master of emotional stage directions. He reaches under an actor's skin to prescribe small, delicate changes in emotion. In considering these unusually descriptive and specific stage directions, it is useful to remember that Long Day's Journey into Night is an openly and very accurately biographical play. Thus, this particular style shows a writer attempting meticulously to recreate emotions and events from his own life.