Roger Ebert and Gene Siskel, who went on the air together for the first time in 1975, have been off the air for a long time now. Siskel died in 1999, and Ebert bowed out in 2011, two years before his death. But, for many people, they remain the very exemplars of film criticism. Fellow-critics still admire their vigorous, wide-ranging discussions while, for the public at large, their thumbs-up/thumbs-down gimmick, which they came up with in 1982, has proved indelible. The story of their rise to fame is told in enticing detail by Matt Singer in a joint biography titled—what else?—“Opposable Thumbs.” For Singer, the critic at ScreenCrush and the current chairperson of the New York Film Critics Circle, the book is clearly a labor of love. He writes that his own aspiration to be a critic was sparked by their show, which he began watching obsessively as a middle schooler, in the early nineteen-nineties.

Singer’s admirably fanatical research renders this obsession tangible. He seems to have absorbed every moment that the duo spent onscreen, whether on their own show or other people’s. (They were Johnny Carson and David Letterman regulars for years). He has combed his heroes’ writings and interviewed their colleagues, friends, family, and fellow-critics. But, more than merely gathering this material, he has thought deeply about it, and the best thing about the book is the way that it highlights some of the basic quandaries that critics confront (or avoid) daily. These fundamental conundrums of criticism involve questions about specialism, authority, personality, art, and business. And, with Siskel and Ebert, these dilemmas came into play long before the duo joined forces on television. Both Illinois natives, the two men came to their critical careers by very different paths, but they had one crucial thing in common: neither had set out to be a film critic.

Ebert, born in Urbana in 1942, was one of those precocious journalists who seem to have printer’s ink in his veins. As a high-school sports reporter, he won an Associated Press prize for professional (not just student) journalists. He edited his college daily at the University of Illinois and, in 1966, having begun a doctorate at the University of Chicago, took a day job as a reporter at the Chicago Sun-Times. The next year, when the newspaper’s veteran movie critic retired, he was tapped as her replacement, having written a few film-related reports. (A publicist recommended him.) In his wise and engaging autobiography, “Life Itself,” he recalls that he didn’t intend to stay in the job long: “My master plan was to become an op-ed columnist and then eventually, of course, a great and respected novelist.” But within a decade he’d become the first film critic to win a Pulitzer Prize for criticism.

Siskel, in contrast, came to journalism more or less by happenstance. Born in Chicago, in 1946, he majored in philosophy at Yale and planned on a career in law and politics. That changed when, needing to avoid the draft, he joined the Army Reserves. He wound up as an Army journalist at a base in Indiana, and, when he got out of the service, in 1969, he took a job with the Chicago Tribune. He’d been there only eight months when the paper’s movie critic took a leave. Siskel, who’d written a few pieces about the movie world, put himself forward and got the job.

Now the two men were in direct competition: doing the same job at Chicago’s two great rival papers. But, as much as they often clashed, there was another key thing they shared: neither was a movie person. Not only was neither a cinema-studies major (such a thing barely existed in their day) but also neither fit the profile of the cinephile, hanging out at repertory theatres and taking sides in the debates then raging over the so-called auteur theory. They were just regular moviegoers who managed to find a journalistic application for their pleasure. Their paths to television were separate but symmetrical, each maintaining print journalism as the solid basis of his activity. In 1973, Ebert hosted a series of Ingmar Bergman films on television, which scored an Emmy nomination. Then Siskel started delivering brief movie reviews on a local station. Then, in late 1975, the Chicago public station WTTW paired them up for a joint movie-review program.

The breakthrough took a while. Though PBS broadcasts in markets such as New York and Los Angeles brought national attention, by 1981, as Singer tells it, Siskel and Ebert were getting frustrated with the over-rehearsed nature of the show. The pair, encouraged by the assistant producer Nancy De Los Santos, decided to give free rein to everything else—the discussions and arguments, the slips and flubs, the spontaneity and authenticity of their interactions. Rather than an entente between rivals, the format became gladiatorial. Critics on TV weren’t new, as Singer notes; most of them would do a few minutes during news broadcasts. Siskel and Ebert were different. Individually, they remained writers first, transferring the essence of their columns to the screen, but the show as a whole was TV first and foremost, with all the showmanship of the commercial medium. Predictably, their burgeoning fame attracted a backlash. In 1990, decrying the state of film criticism in the august pages of Film Comment, Richard Corliss dismissed the show as “a sitcom . . . starring two guys who live in a movie theater and argue all the time. Oscar Ebert and Felix Siskel.” Singer differs: “To a generation of up-and-coming cinephiles, it was their first taste of film school.”

Singer correctly identifies the show’s crucial ingredient: the two stars’ clashing personalities and relentless competitiveness. As rival newspaper critics, they’d been striving to outdo each other for the best part of a decade. Now they were in the same room. In his memoir, Ebert writes:

In the television biz, they talk about “chemistry.” Not a thought was given to our chemistry. We just had it, because from the day the Chicago Tribune made Gene its film critic, we were professional enemies. We never had a single meaningful conversation before we started to work on our TV program.

Ebert’s expansive, jovial character immediately comes across in this memoir and in other published portraits. Siskel remains more of an enigma, because he died young and also because he was a far more private individual, rarely speaking of his personal life. The portrait of Siskel that emerges in Singer’s telling is fascinating. A former Yale roommate declared him “the most competitive person I’ve ever run across—more so than Michael Jordan or Bill and Hillary Clinton.” Ebert said so, too: “Gene was the most competitive man I ever met.” Ebert may have been intensely conscious of a rivalry with Siskel, but Siskel’s competitive ferocity and confrontational chutzpah were on another level. When asked by his first boss at the Tribune what his ultimate goal was, Siskel answered, “Your job.” (When a colleague expressed shock, Siskel replied, “Candor. It is powerful. It knocks people off their feet. They are not used to it. Try it some day. If you’ve got the guts.”)