Union internationals who go on to represent their country at rugby league are a relatively common breed. Union men who win caps for two different countries are not unknown either. But Williams will be the first to play union for England and league for Wales.
Welsh rugby league is of necessity a broad chapel. The squad, since the national side was re-formed last season, has always needed a player or two who qualify through parentage rather than birth.
'The first thing I did when they asked me was to phone my dad,' Williams says, although he had a shrewd idea what the advice would be. 'I think he would have disowned me if I'd said no. He's a fluent Welsh-speaker and virtually a Welsh Nationalist, so I was brought up with all that.'
That upbringing, however, was in Lancashire. Roy Williams was a prop forward with Llanelli who progressed as far as a Welsh trial before switching to league with Wigan, with a specific, if somewhat unusual, object in mind. He used the money to finance his training as a solicitor, which is what he still does in his adopted town.
As solicitors' sons were wont to do, Williams junior gravitated into union rather than league, working his way up through Orrell and Lancashire and, in 1987, beating Rob Andrew to the fly-half shirt to win his four England caps. He then lost his place to Les Cusworth and, with Andrew to contend with as well, he decided that he 'would have to play exceptionally well to have a chance of getting back in'.
Instead of making himself available to tour Australia in 1988, he joined Salford, who had been pursuing him for several years. He then came very close to going to Australia as well. After only five games in his new code, his name appeared in the stand-by squad for the rugby league tour Down Under.
There was a bit of an outcry about such fast-tracking, and Williams had to wait until the following year for his two Great Britain caps, both against France. Since then he has become something of a forgotten man, hampered by injuries, by playing for a club outside the top bracket and by the fact that switching codes at 29 was leaving it a bit late.
'If I had been capped for England earlier, I would probably have switched earlier,' he says. 'That's what I was hanging back for. But it's marvellous now to have a chance to play international rugby again. I know most of the Welsh lads from playing against them in union as well as in league. They are a great bunch and team spirit would never be a problem.'
Williams, a former PE and history teacher who retrained as a physiotherapist when he signed for Salford, never expected to be in the starting line-up in Perpignan. When he gets on to the field - Welsh substitutes always get on to the field - it could as easily be in the pack as in his regular position of centre.
'If they ask me to play in the forwards, all I'll say is 'thank you',' he promises. But his abiding frustration at Salford has been the absence of invitations to play in his old union position of stand-off. 'I know they are entirely different positions in the two games, but I had never played centre at all,' he says. He has enjoyed it, but you sense that he could have enjoyed it more.
'I never fell out with rugby union - how can you fall out with a game that has taken you all around the world? But league is the better spectator sport. Union has moved a long way towards league already and I see no reason why, once union sorts out all the rubbish about not paying its players, the two codes shouldn't draw closer together. Within 20 years, it wouldn't surprise me if we had just one game.'
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