Soon after Bill Robles flipped on the set, he knew these TV producers had gotten it all wrong.

Cuba Gooding Jr., a man of somewhat smaller stature, simply didn’t embody O.J. Simpson, Robles thought. And he should know. Robles had spent long hours staring at and studying Simpson through two Los Angeles trials two decades ago.

“When I first saw O.J., I couldn’t believe how big he was,” Robles recalls of his initial in-person encounter at the 1995 double-murder trial. “What a huge guy. That was my first impression – how easily he must have taken out those two people, especially when he jumped out at them.”

It all comes back to Robles in vivid detail as he watches the FX miniseries “The People vs. O.J. Simpson: American Crime Story,” which will conclude Tuesday night. And if anyone has an expert eye at sizing up people, it is Robles, the current dean of American courtroom sketch artists.

“O.J. was miscast,” Robles said, speaking to The Washington Post this week from his native Los Angeles home. The artist praises the acting ability of the Oscar-winning Gooding Jr., but says there are many talented actors “who could pass for O.J.”

As the 20th anniversary of Simpson’s civil trial nears, when Robles describes what he observed, the listener begins to appreciate that for nearly a half-century, the artist has often had a front-row seat to criminal history.

Robles graduated from the Art Center College of Design in Pasadena, California, in 1961, and spent much of that decade working in many forms of illustration, from advertising to editorial to children’s books. But when a friend’s connection helped land him in the courtroom for L.A.’s 1970 Charles Manson trial, capturing the daily action over nine months for CBS, he got his first taste of rendering legal proceedings under tight deadlines.

He also was set on an incredible court career that has seen him uncannily depict such figures as Patty Hearst, Michael Jackson, the Menendez brothers and Richard “The Night Stalker” Ramirez. Robles’ artworks from such cases are among nearly 100 courtroom sketches recently acquired by the Library of Congress.

His coverage of Simpson was unusual, though, because Robles drew the former NFL star and performer through both the criminal and civil trials in the mid-’90s.

Because the former case was televised, the focus of Robles’ assignment was to render the off-camera jurors – though to guard their anonymity, Robles was tasked with drawing them as faceless. He is so amazingly accurate in his realism, however, that even those sketches landed him in some momentary trouble.

“Judge (Lance) Ito directly subpoenaed me,” Robles said. “I was drawing the jury as silhouetted.” The problem, Ito said: Even his silhouettes uncannily reflected the jurors.

“We had a court hearing ... and CBS sent their lawyer and explained the situation,” Robles said. Ito requested that the artist draw elements like hairstyles a little less accurately, which Robles obeyed for a time “before I eased back into” accuracy, he says. Ito also created a stamp and insisted on personally approving all sketch art coming out of his courtroom.

“I still have art with his stamp on it,” Robles said.

For the civil trial that followed, Robles observed certain facets of Simpson even more fully.

“He was cocky,” Robles said. “He would rap with reporters – he was a very arrogant guy. He even hit on a girl – 18 or 19 years old – who was handing out the press passes. ... Then for a day, that became a story.”

Robles spent so many months living the Simpson cases that he appreciates the FX series as an opened time capsule.

“I’m enjoying it – it brings back memories,” said the artist, whose work is featured in the recent book The Illustrated Courtroom. “I like some of the location shoots, and the court is accurate ... and a lot of the background stuff, I didn’t know existed.”

That’s because Robles’ eye was so often trained on the here and now in the courtroom, on the nuance of a raised eyebrow and sideways smirk.

So given his extreme study of these real-life faces – “only Kato (Kaelin) was simple to draw,” he said – what does he think of the FX casting?

“They nailed Marcia Clark and they nailed Fred Goldman,” he said of the prosecutor portrayed by Sarah Paulson, and the male victim’s father played by Joseph Siravo. “I saw it and thought: ‘Gee, they hired Fred Goldman for this.’”

He also praises the casting of defense attorneys F. Lee Bailey (Nathan Lane) and Robert Kardashian (“too tall, but very good,” he said of David Schwimmer), and said that prosecutor Christopher Darden “was a bit dorkier” than he is depicted by Sterling K. Brown.

He lauds, too, the look of actor Courtney B. Vance, playing defense lawyer Johnnie Cochran, whom he liked sketching: “He had a very interesting face – it was pretty easy to depict.”

But overall, Robles said that the miniseries cast too many actors who don’t look like their real-life counterparts. “It’s very distracting,” he said. “To see (John) Travolta as (Robert) Shapiro is a reach. ... It’s as if they said: ‘Who can we get who doesn’t look (at all) like Shapiro? How about Travolta?’”

Mostly, though, Robles – who is still a working freelance courtroom artist – relishes the opportunity to cover such fascinating cases.

“We’re a necessary evil,” he said. When there’s no camera permitted in the room, “we’re the king of the court.”