Perched on a chair facing an inquisitor, Andy Powell looks as wary as a cat caught in a car’s headlights. He presses his palms down on his knees and keeps his back ramrod straight. He seems to be uncomfortable.

But watch him scoop a rugby ball up in one ham of a hand and run and he appears to be a man in his element. He is happy with the startling speed he can generate from a standing start; he knows that he presents a formidable sight to his opponents; he is grateful to hear the roar of the crowd as they see him taking the game to the other team. This is the quintessential Andy Powell. There have been a few sensations in rugby this autumn but one of the biggest has been Powell thundering upfield while playing No 8 for Cardiff Blues and Wales.

You look at him and wonder, first at his athleticism. At 6ft 4in, he is an inch shorter than Ryan Jones, whom he has displaced in the Wales team. At 18st, he is heavier than Jonathan Thomas, the blind-side flanker for the Ospreys and Wales. Yet he has an explosive power and a powerful stride that few can match, and, as a result, he nearly always makes ground.

“He is very powerful and dynamic,” said Richard Hill, the Bristol coach who had Powell under his wing at Newport. “In addition he can keep going for 80 minutes. Some players have his power but not his stamina, some have his stamina but not his power. He has both. He has got fast-twitch fibres. He is an exceptional athlete.”

Then you look again at Powell, who was 27 in August, and wonder where he has been. How has a man who has played rugby for some of Europe’s best clubs — Béziers, Leicester, Llanelli, Cardiff — taken so long to reach the top? The answer is to do with his personality and maturity, his attitude to training and the way he was handled. It is also down to a lack of confidence on his part and his innate Welshness.

At Newport, Hill found that Powell often tested his patience. “You never knew what he would do next, where he would run,” Hill said. “He was difficult to fit into a pattern. One minute he’d do something brilliant, the next he’d do something terrible and you’d put your head in your hands and say, “What the . . .? He was off the scale.”

Allan Lewis, who also coached Powell at Newport, remembers the player as very personable and willing but not given to taking instruction. “When he was good he was very, very good but when he was bad he was horrible,” Lewis said. “We would tell him the patterns, where he had to be, what he had to do and why he should do it and then he would do something completely different.”

Gwyn Jones, the former Wales back-row forward, has watched Powell for nearly a decade. “Andy is fast, powerful and agile and there are very few players like him,” Jones said. “Speed, power and agility are gifts you inherit. They are not coachable. Andy’s problem is doing the coachable.”

Influenced by his father, Powell left Newport in 2003 and went to Béziers in France, where Richard Cockerill, the former Leicester hooker who was then playing for Montferrand, saw him in a pre-season game. “I thought, ‘Crikey, this guy is a good player,” Cockerill said. “Dean soon came and took him to Leicester.”

But that move didn’t work out either and Powell played only once for the first team. He did, however, turn in a stormer for the Tigers’ second XV against Sale. “He was everywhere, carrying the ball, getting over the gain line,” said Dusty Hare, then Leicester’s second XV manager.

So what happened? “The lad was troubled,” Hare said. “He had a lot of problems back in Wales.” The next move was to Llanelli, and while the Scarlets recognised Powell’s talent and found him to be personally likeable, his unreliability was difficult to deal with. “He kept missing training,” a club official said. “He’d say he’d been to a family funeral. He must have had 40 grandmothers who had died.”

In 2005 he joined Cardiff, at the same time as Xavier Rush, the former All Blacks No 8 who is now captain of the Blues, and Rob Howley, the scrum half who is now one of Wales’s assistant coaches. Powell has found a peace in the Welsh capital, not far from Brecon, his birthplace.

“In the past I think he has thought with his heart, not his head,” Justin Burnell, the club’s forwards coach, said. “His temperament has let him down. Xavier’s absence through injury means Andy has had a decent run and he has got some confidence now.”

At this, the words of Ian McIntosh, who coached the young Powell, come to mind. “He needs to be told how good he is,” McIntosh said. “He is very talented but he lacks confidence. If people criticise him, it is no good.”

Some in Wales see Powell as a show pony, who stars when the ground is firm and thus could be a potent player for the Lions in South Africa next summer but who struggles in defence and at the tackle area. What is more, after his startling performances this season, he will be a marked man. But that is his next challenge, according to Lewis. How will he cope when he has a man on him? “That will tell us how he is developing,” Lewis said.