Appel sought freedom in his art, and in the mid-1940s he developed his characteristic imaginative depictions of humans, animals, and creatures — all with grotesque expressions, large heads, and childishly wide-open eyes. In his quest to avoid the discipline, rules and awareness of adulthood in his art, he drew inspiration from children’s drawings for the spontaneous and naive idiom that pervaded his production. In the 1950s, Appel slowly did away with his at once frightening and happy monsters, leaving only reminiscences of them as detached eyes or outlines of figures. Alongside these movements, Appel experimented with nude studies, caricatured portraits and various artistic media such as assemblages and sculptures formed from wood or waste.

He had an immense need to express himself and an unbridled urge to work. Most often, he let the impulses of his mood control the content of a work, the execution itself becoming an almost bodily act for him. He set down his motif with free, sweeping arm movements and said this of his working method: “When I paint, I do not feel. I paint only with my hands, let them seek for me (…) I work spontaneously, without engaging my brain.” Appel’s wild approach to the artistic creation process was fundamental, and his art is characterised by a raw power that continues to fascinate art collectors around the world.