And all he has to do is make us laugh

Conan O'Brien is accustomed to having big shoes to fill. The dryly self-deprecating comedian has been treading on some of the most hallowed turf in American late night television for the past 16 years, sometimes successfully, sometimes less so, and he knows the peculiar satisfaction of proving his severest critics and detractors wrong over time. Starting on Monday, he will need all the luck he can muster - from his Irish ancestors, or elsewhere - as he takes on the biggest assignment of them all: hosting The Tonight Show, the legendary NBC chat programme whose history mirrors the fortunes of network television itself. The hope is that he will turn into another Johnny Carson, the quick-witted giant who dominated the show for 30 years, from the early 1960s to the early 1990s. The more realistic expectation is that he will at least do nothing to diminish the ratings-leading audience cultivated by his immediate predecessor, the genial Jay Leno. Either way, the pressure is unmistakably on. Like his predecessors, O'Brien will be expected to lampoon and skewer the headline-makers of the day and interview the glittering finest in politics and show business. That, in itself, should not be too much of a stretch. The red-haired, freckle-faced comedian with the outrageous quiff has done much the same since 1993, as host of the programme that follows The Tonight Show each evening, Late Night With Conan O'Brien. He knows how to do goofy banter with the members of the in-studio band. He knows how to be irreverent without pushing the boundaries of acceptable taste. And he knows, above all, how to project his own comic persona while also allowing his guests to shine in their own right. Two things, though, will be very different. The audience will be significantly larger - close to six million a night, compared with fewer than two million for his old show - and consequently just a touch more conservative. He will almost certainly retire some of his regular comic foils like the Robot on a Toilet. The stakes, meanwhile, will be immeasurably higher. The Tonight Show is an institution recognised by every television-watching American and loved by a good proportion of them. People who have paid only incidental attention to his career so far will now be scrutinising his every move. Critics who have previously switched off and gone to bed when they found him uninspiring, or just a little bland, will be shouting from the rooftops if they find fault with him now. Fans who have grown used to his way of running the late-night show will be on the lookout for any sign that he has "sold out" to satisfy the more mainstream audience. As O'Brien himself put it the other day: "For the first time in my career, I'm performing for people that are fully awake." It may not be an exaggeration to say that the very future of network television rests, at least in part, on his shoulders. In an age of cable, satellite and digital media, the original Big Three of American television - NBC, ABC and CBS - are fighting for their lives. Their evening news broadcasts have dwindled under competitive pressure from the cable news networks and the internet. Their schedules of original comedy and drama series have come under assault from reality programming and the edgier, more original material emanating from cable stations such as HBO. That leaves the chat show as the flagship for each network. For now, Leno and David Letterman, on CBS, have done a decent job of keeping themselves current, and popular, and their grateful networks have showered them with hugely lucrative contracts. It became apparent a long time ago, however, that neither man could go on forever, or hope to keep appealing to new generations of younger viewers.

Torn between the merits of keeping Leno in place and finding a replacement, NBC did something rather extraordinary. It offered the job to O'Brien five years ago, giving him an unprecedented amount of preparation time and giving the American public lots of cushion room to get used to the idea of Leno bowing out. Now it is crunch time. Last night, Leno was gracious enough to invite O'Brien on to the show as a guest so he could pass on the baton in front of viewers. On Monday, O'Brien will kick off with what he hopes will be a memorable show, with Pearl Jam, the Seattle grunge rockers, doing the musical honours and fellow comedian Will Ferrell, an old friend from his days on the NBC sketch show Saturday Night Live, helping him out as the first invited guest. After that, his room for error will be minimal. David Letterman will be snapping at his heels at every turn, hoping to reverse the ratings that always put him second best behind Leno. And memories will no doubt be vivid of his less than auspicious beginning as a chat-show host back in the early 1990s. O'Brien, when he started out, was an emergency hire. At the time, Letterman had hoped to take over from Johnny Carson and was so incensed when Leno got the job instead that he quit NBC for CBS. That left a gaping hole in the late-night schedule. NBC offered the job first to Dana Carvey - Mike Myers's sidekick on Wayne's World - then to Garry Shandling, who was just starting his spoof backstage chat show comedy, The Larry Sanders Show. Both turned it down. O'Brien, then working as a writer and producer on The Simpsons, was suggested by Lorne Michaels, his old boss at Saturday Night Live. He had done some stand-up and a little acting, but was known mostly as a writer - which is to say he was essentially an unknown quantity. On his debut show he smartly parodied the media mutterings that he could never live up to Letterman, who had built up a cult following among college students and young people during his 10 years on the air: footage showed him listening impassively to the criticisms, then starting to hang himself in his dressing room. Live on camera, however, O'Brien was plainly nervous and received generally unfavourable notices. NBC refused to give him a long-term contract, putting him on three months' probation, which was repeatedly extended over his first year. The only reason he was not fired was that they had nobody else in line to replace him. Slowly, he learnt his craft. By 1996 - three years in - the show was earning Emmy award nominations. O'Brien grew more confident about making fun of himself - his pale, freckled skin, his red hair, his lanky height - and used a wide variety of props to keep the chat-show format fresh. A celebrity-bashing puppet called Triumph the Insult Comic Dog proved particularly popular, and may well survive the transition to The Tonight Show. He had one show reshot with clay puppets, just for fun. Another episode was rerun at Halloween with all the human figures on camera replaced by skeletons. When his writers stopped working during a strike over the winter of 2007-2008, he made a running gag of the fact he had no material - taking, for example, to spinning his wedding ring on his table to see how long it would stay upright. Nobody doubts O'Brien is smart. Born into an Irish Catholic family near Boston, he went to Harvard, and cut his comic teeth on the Lampoon magazine there. He is clearly popular, both with his audience and with his peers. At 46, he is young enough to bring in the fresh audiences NBC craves. The greatest concern, perhaps, is that his comedy comes over as rather slight - comfortable more than edgy; chuckle-inducing rather than roll-on-the-floor laughter. He was once described as producing a "talk show for the nerdy set". He is unlikely to develop the sort of cult following enjoyed by Letterman in his heyday, or by the new generation of spoof news anchors such as Jon Stewart or Stephen Colbert on the Comedy Central cable channel. Asked how he intended to approach the new assignment, he told one recent interviewer the story of how he first met Johnny Carson, who counselled him: "Just be yourself." Almost immediately, though, he contradicted himself, saying he would not make the show a carbon copy of his Late Night format. "I think people would be disappointed if I didn't reinvent myself to some degree," he said. Clearly, he is feeling the heat. These are nervous times for American network television, and Conan O'Brien is carrying a lot of weight on those tall Irish shoulders. * The National