Former Wallaby back turned commentator Greg Martin described it thus: "Rocky is a real individual, a lone wolf - when Horwill says follow me he actually turns around and asks you to follow whereas Rocky just takes off."
Whether this hits the nail on the head might be open to argument, but you get the general picture.
When the 26-year-old Horwill does stop to think, he doesn't let it show. He is a 2m tall, 117kg, square jawed, open-faced tower of energetic enthusiasm. The big lock is so naturally effervescent that an older player at college nicknamed him after a flamboyant television ad man known as Big Kev, a handle that has stuck.
"I struggle to sit still a lot of the time - I guess that has been me for a long time," Horwill says, although he does a brave impression of a man relaxed during this interview at the Wallabies' Takapuna hotel, days out from the World Cup semifinal.
He was a budding Aussie rules player as a kid, growing up in Brisbane where the success of Queensland and the Broncos stamped the place with league, although it wasn't a sport that interested Horwill.
Back then, he knew only vaguely of Brad Thorn, the Bronco forward who will oppose him in the All Black pack tomorrow night.
Horwill's mother, Jenny, was from Richmond in London, in the shadow of Twickenham, and his dad, Rod - who runs the family's plastics manufacturing business - from Victoria, the heartbeat of Australian rules.
They had moved to Brisbane before James - the middle child of three - was born. Horwill played more than 100 junior Aussie rules games for the Kenmore Bears, playing rugby on Saturday and rules on Sunday for a time.
His idols included Wallaby forwards John Eales and Toutai Kefu, along with Roger Merrett, the Brisbane Bears' stellar full forward.
The frame of an international rugby lock was taking shape in bursts outwards and upwards.
His younger sister, Catherine, is a tallish 1.8m but his brother, Chris, is of average height, his mum "quite short" and his dad "not a big man".
"A cousin is taller than me though, so the genes are in the family lines somewhere," he says.
With Horwill attending Brisbane Boys College, one of the private schools which are the bedrock of Australian rugby, and noting his son was on the "chubby" side, Rod Horwill encouraged him to play rugby.
"Dad convinced me to start playing rugby when I was about 11 and I wasn't that keen on it, to be honest," he says.
From a young age, and being a big kid, Horwill has captained sides, and he has led the Reds since he was 22.
If ever a player has learned how valleys can quickly become mountains it is Horwill, who has been part of the worst and best of it with Queensland.
The nadir came in 2007, when the Bulls annihilated the Reds 92-3 at Loftus Versfeld.
Horwill found it tough to walk down the street after returning to Brisbane, hearing it described as the worst defeat for a Queensland rugby side.
This year, under Ewen McKenzie, the Reds shocked one and all by winning the Super 15 with a wonderful brand of rugby.
"We climbed one hell of a mountain - you feel embarrassed after a performance like the one against the Bulls, you always want to be proud of what you do," says Big Kevvy Horwill, whose advances have included ironing out a kink which involved ironing out a few opponents in his initial seasons in the big time.
"When you have been part of the lowest point and also part of one of, if not the, highest points, you understand what the effort of a group means, that things don't just magically happen."
And they haven't for Horwill. He missed the 2007 World Cup squad, and also had to keep the faith after a serious injury last year. In the second Super game, a victory over the Crusaders in Brisbane, a foot went one way, the knee another and the popping sounds and excruciating pain told him this was no minor matter. Specialists found almost everything in the knee torn or snapped, the good news being that the damage was clean so a major reconstruction would make the joint like new.
On a personal note, he has also dealt with dreadful tragedy, his two best friends killed in a boating disaster just before he made his Wallabies debut on the European tour in late 2006. In a party mood, the pair had arranged to meet Horwill and instead of a taxi took an inflatable boat, which hit another boat in the wee hours. Before every match, Horwill writes their nicknames on his arm tape.
He has already experienced marked sporting lows and highs in a short spell after his shock elevation to Australian captain.
Deans called Horwill aside after the last Tri-Nations game in Durban to tell him he was the new leader, news Horwill had to keep close until the World Cup squad announcement a few days later.
Deans, a big Elsom fan, had helped broker his short spell with Irish club Leinster and thus ensure he returned to the Wallaby fold. The bruising blindside won rave reviews in Europe and made the Wallaby captaincy on his return, replacing Stirling Mortlock and caretaker leader George Smith. But Elsom didn't always find best form, one reason Deans may have made the late change.
Horwill had no warning, being as shocked as the public was to be when he was revealed as the new skipper when the squad was unveiled three weeks before the tournament.
"I didn't have to think about it though," says Horwill, who plays his 33rd test tomorrow, a relatively light history for a captain by modern standards.
"Not many get this chance - I was the 77th person to ever captain the Wallabies and I feel very privileged.
"This is a huge responsibility but also a huge honour that the whole organisation at the ARU respect me that much."
For all the famed Big Kev-style excitability, there is also a theme of stability and level-headedness to Horwill. He left the family home, the only one he has known, just a couple of years ago, and has a "long-term" girlfriend, Lauren Diamond, who hails from Melbourne. Horwill takes the added media duties of being Wallaby captain in his easy stride.
"I don't get too stressed about that - I understand the media are trying to do a job just like we are and that side doesn't worry me at all," he says.
"I don't see anything as a chore - I enjoy being captain, enjoy being part of the team. I like to enjoy whatever I'm doing. That's the most important thing, whether it is rugby, work or anything else."
Horwill immediately led the Wallabies to a Tri-Nations-sealing victory over the All Blacks at Suncorp Stadium, only for a lot of good work to seemingly come undone in a shoddy World Cup pool game against Ireland.
What followed was an epic of sorts, as the Wallabies were steamrolled for long periods by the Springboks in Wellington, yet emerged unflattened to claim a remarkable quarter-final victory. Horwill was caught by a television microphone telling his team, "I'm f***ing proud of you guys."
Had Elsom still been captain, he would have felt this, but might not have expressed it so blatantly.
Yet action still speaks louder than words for Horwill. His confidant is Eales, an always available sounding board for Horwill when he needs a sympathetic, intelligent ear.
Horwill says: "He told me to be a leader not a captain, to lead by what you do and how you go about things."
A curly question: What does this eternal optimist find hardest about being captain?
"Ahhhh ... good question ... I haven't been asked that before. I want to think about that ... let's get back on that one." He doesn't, and was never likely to.
