Ireland coach Joe Schmidt's greatest fear factor is how France can change, like a chameleon, to make magic happen out of nothing.
There are those who have coldly received the latest incarnation of Philippe Saint-André's era from a rather dull and blunt win over Scotland.
Schmidt knows better than that: "It can happen in 24 hours," he warned.
"I think anyone who was at the World Cup last time in 2011 watched France play Tonga and you give them no chance of being in the final, let alone winning it.
"They were incredibly unlucky not to win it and probably deserved to," he said.
The very things that allow them to transform from rudderless to rampant are the unique gifts they take to the table.
What exactly scares Schmidt most? "Their individual abilities to beat us one-on-one, their individual abilities to physically dominate us.
"I don't just mean in the physical, confrontational manner.
I mean to be able to physically beat us to the space with the footwork they have."
The New Zealander pulled up a personal example from his extensive file of three years working at Clermont-Auvergne in the Massif Central.
"I had the luxury of coaching Wesley Fofana for a few years," he said.
"I don't think there's many better than him in world rugby at being able to change an angle whether it is taking a hard line and breaking a tackle or to take an out-line and skin a tackle, so that it doesn't exist."
Fofana is not alone in carrying an every-play danger into the Aviva Stadium tomorrow evening.
"How do we contain Bastareaud?" he asked, neatly avoiding to answer his own question.
Certainly, it was a two-man assignment for Brian O'Driscoll and Gordon D'Arcy in the Championship decider last year.
"That's probably what fear I have, the greatest fear I have, because they do have that talent."
For all of that, there was an intensity about Schmidt in Carton House. France do have a way of focussing the mind.
He also has to contend with the great unknowables.
Always calculated, not even Schmidt can predict with real authority how Seán O'Brien, Jonathan Sexton or Jamie Heaslip will hold up under the unforgiving microscope of international rugby.
"It's a long time since Seán has played with us. It is a long time since Johnny has played full stop. It's Jamie's first game in the championship," he said.
"It's not perfect and our performance no doubt will reflect that to a degree.
"I think, hopefully, we can get some tempo into the game and be effective."
What goes for Paul O'Connell goes for these three. It is not just what they bring individually, but how they lift the confidence of those around them.
"Having those people around they know that those players have been in some tough, tight moments before.
"They have been in some very successful moments as well therefore their decision making, their leadership does add a bit of confidence.
"You grow game by game and these guys, amongst the 130 odd caps that they have amassed, those experiences have built a degree of confidence and a degree of organisation that become a little bit automated.
"They have been in situations before and have worked their way through them or, having not worked their way through them, learned from them."
These are faces you want in an Ireland dressing-room.
There is another rather recent factor in Ireland's favour.
The faithless day when the All Blacks were taken to the wire revived a passionate relationship between the players and their people.
"In my experience, that was the first time, that was unbelievable, that game. I think it was emotional, it was the rollercoaster, the cacophony of noise.
"We have thick windows in the coaching box, thank goodness, because you couldn't hear us crying at the end of the game.
"At the same time, it is the first time that that noise really came into our coach's box.
"There was those last few minutes against Australia. The crowd might not have been playing but it felt like they were part of the defensive line.
"It is something we probably want to re-earn every time we go out and play and the players want to demonstrate again that they merit the support they are getting.
"The support has been an incredibly positive part of those results. It is almost a reciprocal arrangement. They give us a real ballast and we try to deliver a massive physical commitment on the pitch."
The Aviva can become a touchstone for Ireland and a fever against all foreign bodies.