Their work is all but impossible to separate. The writings of Hunter S. Thompson, self-styled creator of gonzo journalism, will forever be linked with the unforgettable drawings of Ralph Steadman, who also featured as a character in Thompson’s berserk narration trips through the dying days of the counterculture.

It’s fair to say that, at least in the days before Thompson’s cult grew, it was Steadman’s illustrations, which often seem simultaneously to have been etched and vomited onto the page, that drew readers to the books, the craziness on the cover being the equal of the craziness inside.

If For No Good Reason, a new documentary about the artist, heavily favors the work he did with Thompson (hardly the dominant part of his output) it’s fair to say that without that association the film wouldn’t exist at all. Incorporating plenty of archival material, it is structured around a visit to Steadman’s British farm by Johnny Depp, who played Thompson in two movies and has a side career as the chief acolyte of the writer, who died in 2005.

Though the septuagenarian Steadman couldn’t be any more gentle or open, Jann Wenner (who as editor of Rolling Stone was instrumental in building Thompson’s legend) says that he was the more truly crazy of the two. Recalling his youth the artist says that “If I ever learned to draw properly I would try to change the world,” a position reinforced by his first visit to Manhattan in 1970, where he photographed Skid Row vagrants.

If Steadman’s illustrations often seem to have been expelled from his psyche in a fit of unquenchable anger, a sequence showing him creating a new work from the beginning is illuminating. He begins by splashing paint onto a canvas from the edge of a brush, then looking to see what might appear from the chaos and gradually teasing it out.

For No Good Reason was clearly devised as side project for the Thompson cult (even the title is a phrase he was fond of using to describe his actions), who will be thrilled to find some previously unseen footage of their icon taken by Steadman. But if it perhaps gives short shrift to the artist who is ostensibly its subject, it still exposes more of his work to a larger audience than he has otherwise had.

Opens Friday June 20 at the Amherst Theater.

Watch the trailer for For No Good Reason