One of the most worrying questions floating around Australia is which Mitchell Johnson will turn up to the Ashes? From casual fans to obsessive followers, to Johnson's team-mates and himself, nobody knows whether he will be that increasingly distant wrecker of South Africa or the erratic slinger who sprayed the ball two years in a row in England.

Johnson landed in the United Kingdom in 2009 with a reputation as the world's most menacing bowler, after breaking Graeme Smith's hand twice in back-to-back series and splitting Jacques Kallis' chin. Instead of angling the ball across the right-hander, he was swinging it in as well, adding some scary bumpers for variety. There were also 33 wickets in six Tests for a bowler living up to the hype heaped on him for a decade.

"He's probably the most hostile bowler of my generation," Damien Fleming, the former Test bowler, told ESPNcricinfo. "Some of the spells I've seen him bowl against good batsmen, particularly from South Africa, have been chilling."

Yet almost two years on from those demolitions, everybody has moved on. Johnson has gone backwards. A sensitive and quiet man, he has to re-programme his character to chirp at batsmen. As 2009 wore on and his form regressed in England, it was as though Johnson was scared by his brutal ability. He lost his line and the lethal instinct he had tried so hard to cultivate.

Having a family battle become public distracted him further, and his deepest trough came at Lord's, where he was either too wide to hit or in the slot. In eight pre-lunch overs on the first day he went for 57 and England sped to 126. It was one of two sessions that ultimately cost Australia the Ashes. When batting in the second innings, Johnson turned his back on a short ball from Andrew Flintoff. It was a sad reaction from a man previously so assured and graceful. The 12th man needed to run out a hug.

Fast bowlers don't have to be rough and mean, but it helps. When the Australians were criticised for being overrated, over-indulged and overpaid early this month it was easy to think of the 29-year-old Johnson. So much has been expected of him but since South Africa, when he was elevated to attack leader in the absence of Brett Lee, he has rarely delivered.

He is the only specialist bowler who was playing in Cardiff, Mohali and Melbourne, those three horror games when Australia were left one wicket short of what should have been comfortable victories. They were the matches that highlighted the differences in attitude and approach between the currently ordained ones and the tough, all-conquering units of the not-so-distant past.

In the Cardiff and Mohali Tests, Johnson had plenty of time to grimace and push and sweat for the breakthroughs that never came. At the MCG earlier this month he smiled at the end of an over of punishment from the soon-to-be-victorious Sri Lankans. Instead of a wicket Johnson returned in the 42nd over to give away 10 runs. Ricky Ponting, a long-standing admirer, needs someone he can rely on in those moments. Johnson has not been that man.

The insecurity of Johnson's performances has led to a debate over whether he can be called the attack leader - and even whether he is worth a place. After a series of serious back injuries growing up, Johnson has done well to maintain his fitness and hasn't missed a Test since debuting against Sri Lanka in 2007. Every other bowler has been dropped or injured and been forced to prove they were worthy of a return, making them stronger and more committed. Johnson has faced that in the one-day side but never in Tests. Former players and those currently involved in the Australian team deliver strong public support for Johnson, knowing how he thrives in an environment in which people believe in him. "I think he is the leader," Michael Kasprowicz, a former team-mate at Queensland, told ESPNcricinfo. "He has this ability of picking up wickets regularly, so I think he is the attack leader. It's a huge part of the Australian bowling line-up that Mitchell fires."

Andrew Hilditch, the chairman of selectors, said this month that Johnson has been "our leading bowler for a long time and one of leading bowlers in world cricket". While Hilditch has chosen to believe in Johnson, his comment also reflects the lack of genuine strike options in Australia and the standard of pace bowling around the globe.

Australian players and officials will defend Johnson's status by pointing to his winning of the ICC Cricketer of the Year in 2009. The award was another example of him being flattered by what he did against South Africa, not flattened for what came after.

A coach at Australia's Centre of Excellence told ESPNcricinfo during the off season that Johnson "needs to be the attack leader" and that he had to prove himself during the Ashes. He said achieving that position would be difficult because Johnson struggles to bowl with the new ball. Not many pace spearheads have had to wait for the openers to finish their spells to have an impact.

When the Australians were criticised for being overrated, over-indulged and overpaid early this month it was easy to think of the 29-year-old Johnson. So much has been expected of him but since South Africa, when he was elevated to attack leader in the absence of Brett Lee, he has rarely delivered

Fleming thinks Johnson is a different bowler at home than away because his armoury is more suited to the local conditions. "He seems to love that there's no pressure to swing the ball in Australia - not with the new ball - and that his pace is a real factor," Fleming said. Johnson has 84 wickets in 17 Tests at 25.59 in his backyard, but overseas the mean is 31.10. Given Johnson's demeanour, it's no surprise that he prefers home comforts.

AFTER THE ASHES Troy Cooley, Johnson's almost-private bowling coach, takes up the job as the Centre of Excellence's head coach. Johnson is effusive whenever he's asked about Cooley's role, but even the man who created such magic with England in 2005 has never got Johnson's wrist in the right position consistently. So much talk for so few outstanding results. Cooley's departure should be a good thing for Johnson, forcing him to think for himself or consider fresh ideas.

Johnson's mind and his left hand are the keys to whether he clicks or cracks. Amateurs find it impossible to understand how a professional athlete, who has only a handful of things to focus on at training every day, can't fix something so minor yet so vital.

When Johnson is working at 80% capacity his action is at its most efficient. His left arm reaches high in delivery, allowing his wrist to sit behind the ball at the optimum angle. In this mode he can make small adjustments to achieve cut and swing. On the rare days when he achieves this at full pace he is the most frightening Test bowler on the planet.

The more common result of him operating at 100% intensity is that his action breaks down, slightly or moderately, and his direction becomes more of a lottery. "When he's feeling good and strong and coming over the top it gives him a greater degree of accuracy with his line," Kasprowicz said. "When he gets tired or is trying to bowl too fast, he can fall away a little bit and the slingy action comes out, and he's a little bit round arm."

In his early Test days Johnson would regularly let go a full and wide ball towards the start of his spell. At the time it was cute, but the lapse is now entrenched in his repertoire. He has also become an expert at picking up wickets with leg-side deliveries. Seven of the 43 Test catches he has sent to the wicketkeeper Brad Haddin have come from this line. When he's going well it's a sign of his golden arm; during his low times it underlines the flaws in his action.

Achieving something from nothing is one of Johnson's great skills. Not many players can turn 0 for 96 into a respectable looking 3 for 120, like he did in the first innings of the Sheffield Shield match against South Australia last month. In Melbourne over the past week he ran through Victoria once, taking four wickets in his final five overs in a stress-busting haul of 5 for 35.

However, a player of Johnson's status has to be more authoritative more of the time. Given his talents, he really should be a great bowler. "He's got the angle of the left-hander," Fleming said. "He bowls a very good bouncer, he has got an excellent slower ball, and he can bowl cutters if the ball is turning. Reverse-swing he's got a good handle on, and I like the way he uses the crease from around the wicket as well." It sounds like the complete package.

Mickey Arthur, who was South Africa's coach during Johnson's reign of terror, said he is the one strike bowler Australia owns. "He probably is a bit hot and cold at the moment, by his own admission, but he's got that X-factor," Arthur said. "He could go through a spell where you're looking at it thinking, 'that was ordinary', but then he can bowl one spell that actually wins you games."

The match-turning success hasn't happened much lately. Often it's hard to work out if he's a good bowler or a lucky one.

Overall, Johnson has 166 wickets in 38 Tests at 29.06, which are decent numbers. His average is slightly better than Lee's, but worse than those of Jason Gillespie, Merv Hughes, Craig McDermott and Jeff Thomson, bowlers who sit below the rung of truly great Australian fast men. Yet Johnson has played in only eight victories in 23 Tests against the top four teams: India, Sri Lanka, England and South Africa.

Australia have been in transition throughout Johnson's three-year career and it is not his fault that the outfit has faltered so often when running into in-form opposition. But he has been a heavy contributor to the side's status as the game's No. 5 outfit. Australia desperately need to know what they can expect from him.

Peter English is the Australasia editor of Cricinfo