Spiegelman's perceived audacity in using the Holocaust as his subject was compounded by his use of comics to tell the story. The medium was viewed in the English-speaking world as being inherently trivial, thus degrading the subject matter, especially as he used animal heads in place of recognizably human ones. Funny animals have been a staple of comics, and while they have traditionally been thought of as being for children, the underground had long made use of them in adult stories, for example in Robert Crumb's Fritz the Cat, which showed that the genre could "open up the way to a paradoxical narrative realism" that Maus exploited.
Ostensibly about the Holocaust, the story is entwined with the frame tale of Art interviewing and interacting with his father. Art's "Prisoner on the Hell Planet" is also encompassed by the frame, and contrasts visually and thematicically with the rest of the book as all the characters are in human form in a surreal, German Expressionist woodcut style inspired by Lynd Ward.
The line between the frame and the world is bolded by comments such as when Spiegelman, neurotically trying to deal with what Maus is becoming for him, says to his wife, "In real life you'd never have let me talk this long without interrupting." When a prisoner the Nazis believe to be a Jew claims to be German, Spiegelman has the difficulty of whether to present this character as a cat or a mouse. Throughout the book, Spiegelman incorporates and highlights banal details from his father's tales, sometimes humorous or ironic, giving a lightness and humanity to the story which "helps carry the weight of the unbearable historical realities".
Spiegelman started taking down his interviews with Vladek on paper, but quickly switched to a tape recorder, face-to-face or over the phone. Spiegelman often condensed Vladek's words, and occasionally added to the dialogue, or synthesized multiple retellings into a single portrayal.
Spiegelman worried about the effect that his organizing of Vladek's story would have on its authenticity. In the end, he eschewed a Joycean approach and settled on a linear narrative he thought would be better at "getting things across". He also strove to present how the book was recorded and organized as an important part of the book itself, expressing the "sense of an interview shaped by a relationship".
The story is text-driven, with few wordless panels in its 1,500 black-and-white drawings. The art has high contrast, with heavy black areas and thick black borders balanced against areas of white and wide white margins. There is little gray in the shading. In the narrative present, the pages are arranged in eight-panel grids; in the narrative past, Spiegelman found himself "violating the grid constantly" with his page layouts.
Spiegelman did the original three-page "Maus" and "Prisoner on the Hell Planet" in highly detailed, expressive styles. Spiegelman initially planned to draw Maus in such a manner, but after initial sketches he decided to use a pared-down style, one little removed from his pencil sketches, which would be more direct and immediate. Characters are rendered in a minimalist way, with dots for eyes and slashes for eyebrows and mouths, looking "as if they were human beings with animal heads pasted on them". Spiegelman wanted to get away from the rendering of the characters in the original "Maus", in which oversized cats towered over the Jewish mice, an approach which Spiegelman says, "tells you how to feel, tells you how to think". He preferred to let the reader make independent moral judgments. He drew the cat-Nazis the same size as the mouse-Jews, and dropped the stereotypical villainous expressions. The contrast between the artwork in "Prisoner on the Hell Planet" and Maus drives home the effectiveness of the simpler artwork—"Prisoner" is alienating, while Maus is more inviting, encouraging deeper contemplation and understanding.
Spiegelman wanted the artwork to have a diary feel to it, and so drew the pages on stationery with a fountain pen and typewriter correction fluid. It was reproduced at the same size it was drawn, unlike his other work, which was usually drawn larger and shrunk down, which would hide defects in the art.
Spiegelman has published articles promoting a greater knowledge of his medium's history. Chief among his early influences were Harvey Kurtzman, Will Eisner, and Bernard Krigstein's "Master Race". He acknowledged Eisner's early work as an influence, but he denied that Eisner's first graphic novel, A Contract with God (1978), had any impact on Maus. He cited Harold Gray's comic strip Little Orphan Annie as having "influenced Maus fairly directly", and praised Gray's work for using a cartoon-based vocabulary, rather than an illustration-based one, for telling his stories. Justin Green's Binky Brown Meets the Holy Virgin Mary (1972) inspired Spiegelman to include autobiographical elements in his comics. Spiegelman stated, "without Binky Brown, there would be no Maus". Among the artists who influenced Maus, Spiegelman cited Frans Masereel, who had made an early woodcut novel called Passionate Journey[f] (1919).