Good game, Nigel, though I'd say that it points out how thin the lines can be drawn, and how grey the distinctions between minor parts and supporting work can sometimes be. I can understand why often soap supporting actors can get promoted in status because there is a sense in which most soap acting is kept close to the actual personality of the actor (with a few honorable exceptions - June Brown, for instance, is a formidable character actor of many, many years standing), and the demand for range is not as important as the need to portray a character in an honest, straightforward way.
Being a Lisa Riley or a Bill Tarmey (i.e. a longstanding supporting artist of many years standing ultimately promoted to lead actor) is not quite the same thing as being an aspirant lead actor who spends early years doing occasional support work to earn their dues (as, I imagine, was the case for e.g. Keira Keightley - and, it could probably be said of most of the leading actors in our business that they all have at least one or two supporting credits from early in their career when they were unknowns just starting out - Ewen Macgregor was, indeed, once 'Man in Phonebox' and so on).
With that said, even many 'habitual' supporting artists who go on to mainstream success are more well rounded performers than might at first be apparent. Bill Tarmey, for instance, was not a totally unknown SA even when he was first employed on Coronation Street; he was, I believe, a locally known singer/entertainer, who worked the club circuit, and so the producers were probably well aware that he was a talented performer and comedian, with experience of working an audience, already - therefore asking him to step up the game was hardly a high risk strategy. Equally (and I am going only on vague recollection of the character here, as I wasn't born at the time!), I believe Jack Duckworth started out as a character who was, you've guessed it, a salt-of-the-earth club singer type - so they asked Bill to play (initially, at least) what he knew best, and got an accurate portrayal into the bargain!
To make what might be a ridiculous comparison - one of the best known 'unknown' success stories I can think of is that of Harrison Ford, who was, so it's said, working as a carpenter on George Lucas's sets, when Lucas elected to cast him in a lead role (not in 'Star Wars- that came later) but in 'American Graffiti'. This is true, as far as it goes, but the story slightly underplays the fact that Ford was also a well trained actor who was paying his bills by working his additional occupation of carpenter at the time. No doubt when he was screen tested, he did it as only a pro would know how. Similarly, one of the heroes of my youth, Doctor Who's Tom Baker, was (relatively) notorious (in some circles at least) for having been chosen to play the Doctor, having been 'discovered' working on a building site - this story was again true, as far as it went - Baker's career had stalled to such an extent at the point he was cast that he had started doing labouring work to earn some extra cash. However, the fact that he had also racked up a large number of low budget features, and worked with a variety of significant directors and performers for about half a decade before he took on his most famous role, is convieniently ignored.
It's a thin line as I say - Michael Caine tells an interesting career story in his famous book on film acting (if you read between the lines). On the one hand, he makes the typically determined statement somewhere that 'If you choose to act in support, you'll always stay a supporting actor' (or words to the effect). Yet, you can tell from his own words that the majority of the jobs he did for his first decade in film were, basically, support work of exactly this kind. He may never have been an extra, as such, but he was habitually given parts of one or two lines, which weren't much more prominent. As he tells it, his biggest break, 'Zulu', came only when the casting directors, who felt he was wrong for the part of Gonville Bromhead, had their first choice fall sick, and because of the tightness of the shooting schedule, had to bring Caine in to take the part. Of course, it went on (justifiably) to make his career. Presumably, what he was doing was always shooting for the bigger auditions and never quite winning them until the day he finally fell lucky. And that *is* the sort of success story we can all take seriously.
