“I really thought what I would do if I learned to draw properly, I would try to change the world,” British artist Ralph Steadman narrates at the start of For No Good Reason, the documentary made about his laudable life and career that premiered on Friday in London as part of the British Film Institute’s London Film Festival. “It took Charlie [Paul, the director] 15 years to make,” Steadman told the Hollywood Blog of the film. Using grainy archival footage paired with present-day scenes—and narrated by close friend Johnny Depp—Paul captures the artist’s admirable convictions and lifelong fight against injustice.

“You should be as provocative as you want to. [Artists today], they’re far more interested in their style than their content. They don’t have the anger,” Steadman told the Hollywood Blog. It’s a point that informs the film like a song’s refrain, how Steadman uses his cartoons and graphic art like a “weapon,” how he has a “darkness” inside. In the film, Steadman says of his comics, “I wanted to get it out of my system—it was something inside me waiting to erupt.” And erupt it did, particularly once Steadman was paired with Hunter S. Thompson. Although in the film Steadman describes their coupling as like “chalk and cheese,” both he and Thompson finding the other “entirely ridiculous,” it proved one of the most fruitful and sensationalist artistic partnerships. Steadman provided the illustrations to Thompson’s 1972 book Fear and Loathing in Las Vegas, among many other creative collaborations.

“The pictures weren’t about the story; they were a reflection of the story, what was in Hunter’s mind,” Steadman explained in the film. This prescience is Ralph’s great skill, to reach inside the mind and unveil something that no one else could quite see was there. “I go out of my way to make something that is as unexpected to me as anyone else,” said Steadman of his work when speaking with the Hollywood Blog. Or as Rolling Stone editor Jann Wenner eloquently puts it when interviewed by Paul in the film: “It’s what we’re thinking at the back of our heads, but not able to get at.”

“Art is just tricks. It started as a blank sheet of paper and became more than that. Chance and accident take over. Consciously I do nothing,” Steadman says in interview footage. Whether that’s strictly true, only Steadman could know, but his work does begin with a paintbrush and an elegant flick of the wrist. He then continues adding layers of paint, before stripping away layers to unveil the story beneath. And the name of the film itself comes from his reliance on chance—that one of the best reasons to do something is “for no good reason.”

Steadman has always looked to artists who posed questions of the world; to that end, he refers to Rembrandt as the “most inquisitive artist,” based on his fascination with self-portraiture and documenting his own decay. Picasso, meanwhile, is characterized as the artist who influenced him most, though Steadman became so entranced by Leonardo Da Vinci that he wrote a book about the artist—from the first person, no less. In 1982 he “becameLeonardo. I would write it in the first person. I recognized something of me in him. He was fascinated by why things work. Everything he invented was the result of thinking of life itself,” he explains in the film.

Remaining entirely forthcoming, Steadman summed up hisraison d’être to us like this: “Some people really deserve a custard pie. If you’re not going to shock or enlighten, I don’t really see the point.”