Amid all the praise for JJ Abrams' new Star Trek revamp, there is little mention of the man who, for 40 years, practically WAS Star Trek: William Alan Shatner.

This is, in one sense, understandable. The brand has moved on. Abrams was never a Star Trek fan; like an advertising creative, he has simply rebranded his product, even writing the Star Trek characters into an alternate dimensional universe so he could more or less ignore the franchise's last 40 years. But if there was room for Leonard Nimoy to reprise Spock, why not for Shatner?

Shatner, personally, is OK he isn't in the new film. Shatner is happy the franchise is succeeding. Shatner didn't watch Star Trek much either, now you mention it. Shatner is perfectly comfortable no longer being Captain Kirk. Shatner knows exactly who he is: Shatner.

Shatner didn't always know he was Shatner. He described his epiphany to US talk show host Conan O'Brien about a decade ago. Everyone at Star Trek conventions was always talking about "Shatner this" and "Shatner that", Shatner recounted. They were going on as if Kirk/Shatner was a hero with magic powers, some sort of binary god. Who WAS this mythical figure, this "Shatner", Shatner wondered. Then, he said, it dawned on him. HE WAS SHATNER.

From that moment on, Shatner has been Shatner as intensely as any celebrity has ever inhabited a personality. He is one of the few famous people to embrace the fact that his image is nothing more than a plaything of the public, a sort of reality TV star before the genre existed. In everything he does, he embodies the public's conception of him as a hack who takes himself too seriously, a living joke – from his famous "Get a life" Saturday Night Live skit in 1986, through his adverts for Priceline.com, to his brilliant album Has Been and his role in the film Free Enterprise, in which he played himself as a Shatner burning with ambition to stage a rap opera of Julius Caesar starring himself in all the male roles. This endearingly zen-like acceptance – so diametrically opposed to, say, Christian Bale's huffy feud with his own image – allows Shatner to be simultaneously ironic and earnest in a way that has really come to resonate in the 21st century.

So of course he's not in the new film. There'd be no way to include him that wouldn't trigger our awareness that he was Shatner; that wouldn't turn even a tiny cameo into The Shatner Show. (Abrams says Shatner wanted to dominate the film; Shatner claims he was never even asked. Both are probably true.) The TV show Boston Legal only gets away with casting Shatner as Denny Crane by letting him "do" Shatner. He won an Emmy for the role.

His first major act as Shatner was his most inspired. He appeared on stage at the Science Fiction Film awards in 1978, under a spotlight, cigarette in hand, to perform Rocket Man, experimenting for the first time with Shatner as Shatner, giving himself three alter egos: pensive smoker, shoulder-rolling martini lizard, skyward dreamer. Ever since, he's no longer really been able to act. The one thing he CAN do, better than anyone in this galaxy, is play Shatner.

One of my favourite Shatner moments is an audio recording in which a young director, recording Shatner doing a 10-second introduction to a science fiction radio programme, has the nerve to tell Shatner he isn't reading his lines right – and actually reads them himself, to show Shatner how. Shatner obeys, mimicking the producer's hammy, exaggerated, "Shatner-like" delivery until the producer's voice is breaking like the fast food employee from the Simpsons: "Um, Mr Shatner, please, I'm sorry ... ". Do Shatner better than Shatner, indeed. The kid's lucky Shatner didn't kung-fu chop him.

The episode highlights Shatner's legendary ego – which Shatner, of course, is the first to acknowledge. His book Star Trek Memories is full of stories about how the original Star Trek cast "despised" him. ("It was just that I must have been rather ruthless about wanting the show to be at its best," he cheerfully told Conan in 1993, "and if lines were to be cut, well, they were somebody's else's lines". This comment followed close on the heels of his complaint that the angle of Conan's desk "upstaged" him.)

At his own Hollywood roast, he rode in on a HORSE, thereby undercutting all the skewering of his ego that was to come – making it all seem so obvious and toothless and even spiteful (George "Sulu" Takei's delivery of the inevitable "Fuck you and the horse you rode in on" coming off as the 40-year culmination of a hidden hatred, which it probably was). When the customary time came for Shatner to turn the tables on the roasters, they were already cowering, like a boy who throws a rock at a car only to mouth "Oh shi..." when the copper gets out of the driver's side. Shatner destroyed them. No one makes fun of Shatner like Shatner.

His favourite targets are fans who don't get the joke. In his SNL skit, appearing before a crowd of stereotypical Trekkers (including Dana Carvey in an "I Grok Spock" T-shirt), a meaty-looking Shatner shocks them: "Get a life, will ya people? For crying out loud, it's just a TV show. I mean look at you, look at the way you're dressed. You've turned an enjoyable little job that I did as a lark for a few years into a colossal waste of time!"

Naturally, as one of the few celebrities who has internalised the knowledge that he's a fictional construct, he's a huge sell-out. His long-running campaign for the travel website Priceline.com specifically trades on the Shatner persona: his inflated ego (a coterie of babes dressing Shatner in furs and ushering him into a limo), his corny sense of humour ("They called you a birdbrain, but it's not true," he tells his pet falcon in one out-take), but mainly his willingness to be a corporate shill in the first place.

The ads were some of the first to employ what has become a common comic trope: a dorky white person using black American slang not to sound cool, but to emphasise their own dorkiness. "You want some of this?" Shatner says. "Then you know what to do, dogg: bust a move!". As usual, however – and just as in the Julius Caesar rap opera – Shatner takes the joke one further: he says "bust a move" like he believes it. That's a lot of levels. That's endless-bathroom-mirror territory. And it was a decade ago. Even better, he lives the joke. You can't get him to STOP pitching Priceline. He brings it up at almost every appearance.

His greatest artistic achievement is the gloriously titled Has Been, produced by Ben Folds, an album with two particularly inspired tracks:

The first is arguably the definitive version of Pulp's Common People, in which Shatner's world-wise delivery makes Jarvis Cocker sound like a 15-year-old who's never had to wash his own socks. Like all Shatner does, the song combines the knowing self-awareness we've come to call "irony" – echoes of Kirk/Shatner wishing he could "be like common people", knowing he is doomed to be marooned by his greatness – with a genuinely uplifting emotion in which Shatner reaches over the battlements we erect around our hearts in public to convince us that Shatner IS just like us, which of course he absolutely is: a cheesy dad, a mild vulgarian, a heartsick widower.

The second, I Can't Get Behind That, features Shatner having a shouting match with Henry Rollins about how modern life pisses them off; no more needs to be said.

Shatner is fully aware of what he's doing, though he's not particularly articulate about it, like the fish who doesn't know he's breathing water. Talking to Larry King about the Priceline ads, he said: "Oh, I had fun with them, but you have got to be careful because it's fun. The fun is the reality of it and yet is the unreality of it. So that line is very, very hazy." (King's response: "You are a very special guy, William Shatner.")

In 1999, Shatner found the corpse of his wife, an alcoholic, floating in their swimming pool. The tabloids shouted. Shatner said his life "ended". He struggled with the grief, was saved by remarriage, and can't believe his luck. "I'm consumed, maybe, with the idea of death, the fear and the mystery of it," he told King. "Life is a joke. We're born and we're going to die. We may die on our second breath."

William Shatner may be a living joke, but his dignity, not to say his genius, is that he's the one telling it.