His starship captaincy may be history and his role as Boston Legal's Denny Crane about to end but William Shatner is not slowing down, reports James Rampton

Does an aura of youthful vigour still cling on to William Shatner? You bet. The former Star Trek star may have reached the grand age of 78, but he's still attacking life with the same gusto that his alter ego Captain James T. Kirk used to take on the Klingons.

"There has been no slackening off in my life," announces the actor, who is still as fit as a butcher's dog and as a hobby breeds, shows and rides American saddlebred and quarter horses. "My wife and I still ride all the time. It's fine getting older, as long as you keep going. Should I ever pause and look at myself, reality is liable to creep in. So at the moment, I'm in total denial."

And he's all the better for it, exhibiting a lust for life that would exhaust a man half his age. Although somewhat heftier than in his sleek Star Trek heyday he still looks exceptionally good for his age. His skin possesses a polished sheen and his hair still has a sublimely healthy appearance, although he baulks at questions about whether it is a wig. "It's like asking somebody, 'did you have a breast implant?', or, 'when did you get your lobotomy'?"

Now, as he approaches his ninth decade, Shatner is embarking on a fresh career as a chat-show host. In Shatner's Raw Nerve screening on the Bio Channel in the US he interviews stars such as Frasier's Kelsey Grammer, the courtroom TV show host Judge Judy, and in the first episode he chats to his fellow Star Trek alumnus Leonard Nimoy.

A star of equal, if not greater, magnitude than his interviewees, Shatner is able to get away with more personal questions. In chatting to his former co-star he elicits touching revelations about how Nimoy cried when his grandfather urged him to defy his parents' wishes and pursue his dream of becoming an actor.

Nevertheless, Shatner admits that he was initially sceptical about the concept. "My first reaction was, 'What do we need another talk show for?'," recalls the performer. "I didn't want to do anything that smacked of tabloidism or made the interviewee uncomfortable. I wanted to go deeper by making it like the sort of good in-depth conversation you'd have over a drink with a friend. I want to leave feeling I have touched someone, not exploited them, just exposed their humanity."

Shatner's new role as a chat-show host follows on from his previous incarnations as an actor, poet, producer, director (Star Trek V: The Final Frontier), writer (of the best-selling Tek War books) and singer - who can forget his spoken-word rendition of the Beatles' Lucy in the Sky with Diamonds or Bob Dylan's Mr Tambourine Man?

Above all, Shatner has maintained his position as one of the most iconic figures in post-war popular culture. As proof of his stature, a 2005 TV documentary explained How William Shatner Changed the World, and the Shatner Building at McGill University in his native city of Montreal is named after him. He has popped up in The Simpsons and, when at a key point in Fight Club, Edward Norton's character is asked who his ideal opponent would be, he gives the answer: "Shatner."

Perhaps he has lasted so long because he has never become too angst-ridden about his status, although he confesses that he was once. "I used to think that Star Trek had endured so well because it was all about me. But over the years it has been slowly revealed to me that it wasn't. That's been one of the biggest disappointments of my life. That," he deadpans, "and the death of a pet."

So where does this admirable willingness to send himself up stem from? "It's important for me to have a sense of humour about what I do," reflects Shatner, who has in the past five years taken leading roles in and won Emmys and Golden Globes for the hit TV series Boston Legal and The Practice. "My philosophy is that you might as well be light-hearted about yourself, because whether you're light-hearted or serious about yourself, the same fate awaits us all."

The performer displays comparable equanimity when asked about Star Trek, the show he was involved with for more than 30 years, over 95 episodes, seven movies and countless spin-offs. Gene Roddenberry's sci-fi show, which first aired at the height of the Vietnam War in 1966, painted an idealistic picture of the beneficent influence of American imperial power right across the galaxy. It also spawned more timeless characters and catchphrases than any other TV series:

"live long and prosper", "beam me up, Scotty" and "it's life, Jim, but not as we know it" to name but a few.

Star Trek has some of the most passionately committed fans on the planet. Shatner could make a decent living just by attending all the conventions that take place around the world but he'd rather draw a line under his years on the USS Enterprise.

He's also eager to underscore that, despite what many diehard Trekkers imagine, he is not Captain James T. Kirk - and most assuredly does not emulate his alter ego's over-dramatic enunciation. "I've never had green alien sex, though I'm sure it would be quite an evening. And when I speak, I never, ever talk like. Every. Word. Is. Its. Own. Sentence."

"I'm very happy with the success of Star Trek, but it's long gone in my mind. It's endured so well because we postulated that a world will exist in 300 years' time. That's huge because there's every reason to think that it won't."

Shatner, who is happily married to Elizabeth - his previous wife Nerine died in a tragic accident 10 years ago - gives no indication of slowing up now. He is planning another series of Shatner's Raw Nerve, is about to publish four new comic books and has made a documentary. William Shatner's Gonzo Ballet is a behind-the-scenes film about a production of Common People, a ballet created by Margo Sappington in Milwaukee and based on Has Been, the self-deprecatingly named album Shatner made in 2004 with the musician Ben Folds.

So does he never feel like winding down and putting his feet up? "No, I want to wind up and put my feet down," Shatner replies. "I'm still feeling incredibly energetic and enthusiastic about life. I still think I'm 32."

Who: William Shatner, the former Captain Kirk and soon-to-be-former Denny Crane

What: Boston Legal's last show ever, TV3, 9.30pm, April 28