William Shatner still feels the loss of his Star Trek co-star Leonard Nimoy keenly. Sometimes, he moves on horseback as a skilled equestrian. "I rode competitively this week against 100 competitors nationwide. I came in third. At 84. I'm riding better now than I ever have." Shatner's body of work is fascinatingly diverse, despite the prevailing idea that he played only one role for most of it. In truth, his career spans the Broadway stage – with roles in The World of Suzie Wong and A Shot in the Dark with Julie Harris – to countless television series, from the medium's infancy (77 Sunset Strip, The Twilight Zone) to its creative height (The Practice, Boston Legal). The last earned him two Emmy Awards and a Golden Globe. Back in 1968, Shatner released a spoken word album, The Transformed Man, which became a subject of some mirth among fans. It might have been consigned to the dustbin of musical history were it not for the fact that the cult following it gained gave rise to three more albums: Has Been (2004), Seeking Major Tom (2011) and Ponder the Mystery (2013). Shatner now seems to be at something of a crossroads, both full of life and, perhaps momentarily, curious about death.

William Shatner (centre) won two Emmy Awards and a Golden Globe for his role as Denny Crane in the television drama Boston Legal. As Captain Kirk, he once faced death on the side of Yosemite's spectacular jewel, El Capitan. "Even as I fell, I knew I wouldn't die, because the two of you were with me," the then-58-year-old Captain Kirk told his friends, Dr McCoy (DeForest Kelley) and Mr Spock (Leonard Nimoy​), before adding: "I've always known I'll die alone." It is a pensive moment that seems to have an echo in the present, as Shatner professes to be coming to terms with his own life, and his fear of death. Three months ago, he rode a motorcycle from Chicago to Los Angeles – a distance of more than 2811 kilometres. What he came away with, "ultimately, philosophically", he says, was the frightening notion that his life was in jeopardy for eight days, as he travelled along the United States' clogged highways.

I've got all the hungers and desires and emotions that I had when I was 20 – everything. "I was afraid of dying," he says, noting that the fear of dying is supposed to open one to joy, making one more conscious of life. And so it did. "Day after day ... I'm looking around and I'm smelling life, the sweetness of cut grass, cattle cars going by, and seeing the horizon and feeling the wind and the heat and the cold, every sensation," Shatner recalls. "The immediacy of life, and how fragile life is, stayed with me." Fate then sent him to Asia, on tour, and an unexpected but meaningful brush with Buddhism. "Buddhist philosophy ... to be simplistic about it, [is about] staying in the moment, living life as though you are going to die, the very things I was going through on the motorcycle trip, and I began to read more about it," he says. "Bringing it to today and the [live] show ... intuitively, I have subscribed to that philosophy, saying yes to the joy of life, feeling it at every moment." This year, perhaps more than any of those preceding it, has altered Shatner. The loss of his former co-star and friend Nimoy weighs heavily on him. The pair co-starred on Star Trek – one as the emotional man of action, the other as his logical, ever-present counsellor – and enjoyed in its aftermath (and many return engagements) a lifelong friendship. Nimoy was there, Shatner remembers, through his darkest moments, including the death of his father, Joseph, and the death of his third wife, Nerine. In losing Nimoy, Shatner fears he has also lost the man who "validates so many memories".

"He and I shared so much on a personal level, both as actors and as friends: children, divorce, death. He was besides me when my father died, when my wife died, he was a support. Just one thing after another." For Shatner, Nimoy's testimony of the events they shared was a corroboration of his own memory. With Nimoy's passing, Shatner dwells sadly on the idea that he must carry those shared memories alone. "I'm tearful at the sadness of his passing, but I'm also reluctant to let go of all the memories," he says. "He was the only one to validate all those particular memories. And he's gone. That's the sadness of losing a deep, deep friend like that. And how many times do you make them? I'd never had a friend like that; I'll never have one now. It's like your great love, it's gone and it's over, and that's the sadness." What remains, though, is a sparkling life. Shatner lives in Los Angeles with his fourth wife, Elizabeth. He has three daughters – Leslie, 57, Lisabeth, 55, and Melanie, 51. He also owns a 150-hectare farm near Versailles, Kentucky, named Belle Reve. When he is not writing or touring, he breeds and shows horses. Although the role of Captain Kirk seemed to coat him in an aura of American masculinity, there is something gentle about Shatner. He is direct, but never forceful. His manner is soft and charming.

As a father, he says he was gender blind. "I used to be a terrific skier ... I stopped skiing because I fell and I couldn't get up – the snow sucked me in. But I got all three daughters skiing. I pushed them down black diamond runs. Some of them took off their skis and walked down, some of them tried, but I pushed them, because that's what I would have done with myself. I thought, you're girls, but so what? "And some of that rubbed off," he adds, laughing. "I have one daughter who's an adventurer – likes to go everywhere; I have another daughter who doesn't want to do it, who is happy to be herself with her children; and I have someone in between. So I pushed them, I pushed my daughters, and my grandkids are great athletes, maybe partially because of that." After a long and varied career, Shatner is experiencing something of a third incarnation. The earlier man, off the back of Star Trek, had a patchwork quilt of work that included some memorable turns, including a role in The Twilight Zone episode Nightmare at 20,000 Feet. The second Shatner was a serious, award-winning television actor, whose performance as attorney Denny Crane earned him the affection of one of the finest writers in television, David E. Kelley, and the kudos of his peers. And the third? A spoken-word star whose speaking tour fills stadiums and arenas with an audience hungry for laughter, wry wit and reminiscence. It seems a long way from the world of the younger Shatner who came to Australia in 1969, as many Hollywood stars did back then, to attend the TV Week Logie Awards. He loved it so much he returned half a dozen times, most recently for last year's Comic-Con. "It's a glorious country," he says. An earlier version of his live show came to Australia, but it has been tweaked significantly, weeding out elements Shatner did not think worked.

"I do a lot of documentaries, I've been writing and directing a ton of documentaries, and you've got to [be able] to say, this isn't working," he says. "You have to fail." The result is Shatner's World, which The New York Times described as a "chatty, digressive, and often amusing tour of [an] unusual acting career". Although he spent much of his career in front of a camera, Shatner is at home on the stage. "As long as you know your words," he says, "that's the heart of it. Am I going to blank out? Go dry? Everyone goes dry, [Laurence] Olivier went dry and didn't go back on the stage for 10 years. I understand that completely. Doing an hour to an hour and a half on stage is a big deal. That is a lot of words to remember." Equally, comedy is a comfortable space for Shatner. "I've been getting laughs since I was six," he says. "I understand humour. I may not be funny at times, trying too hard, but I understand the function of the laugh, where a laugh is ... Getting a laugh is like finding a gold nugget." One thing is certain: with thousands of eyes staring back at him in the darkness, and millions more connected to him because of a single character whose courage, humour and humanity seemed never to fail him, William Shatner will never die alone.

Shatner's World ... We Just Live in It is at the State Theatre, Sydney, on October 16 and the Arts Centre, Melbourne, on October 17. See shatnersworldtour.com.