[6] However, the artist's approach changed after he had a self-described "epiphany" at a 1982 show of still lifes by Gordon Cook at Charles Campbell Gallery in San Francisco.

"[4][3] The subject matter of his paintings has remained largely consistent since the early 1980s: San Jose Museum of Art curator Susan Landauer wrote in 2007 that "for the past fifteen years, (Diehl's) repertoire has consisted mainly of hardcover books, typically on art or other subjects... often accompanied by some related iconic element.

"[4] Los Angeles Times art critic Leah Ollman writes that Diehl "practices painting as an act of homage," citing "visual quotes" and references to artists including Morandi, Rothko, Zurbaran, Ingres, Goya, Modigliani, Demuth and Joan Brown in a 2004 review of Diehl's paintings at Schlesinger Gallery.

[8] Diehl's work also incorporates references to artists in other disciplines, including musicians such as Dave Brubeck, Thelonious Monk and Billie Holiday.

[16] In 2013, Diehl created a nine-by-eleven-foot mural entitled Still Life with Billie Holiday, a "transmedia" work with embedded RFID tags, at 240 G Street in downtown Davis, California as part of an interactive art walk installation organized by John Natsoulas Gallery.