England go into Saturday's quarter-final as favourites to progress, having gone through the pool stage unbeaten.
France, on the other hand, scraped through despite defeats to New Zealand and Tonga - while Marc Lievremont's side have lost three of their four previous World Cup encounters with England.
However Johnson insists his side must be as mentally sharp as they are physically tough if they are to combat the unpredictable French.
"The best plan doesn't survive first contact with the enemy," Johnson said.
"We know France can underperform but we also know they can produce games that are almost unstoppable, as we have seen at World Cups.
"You absolutely expect anything and for it to be of the highest standard. Anything can happen and we have to deal with it mentally.
"If you don't deal with it mentally you can't deal with it physically. What if they get an interception and we are 10-0 down after 10 minutes? We fight back. That means getting three, six, nine points or you hopefully slam a try in, two.
"What if you get a big score and you are ahead? That can also be difficult to handle, it can freeze a team. You need that adaptability and that thought on the field.
"We need to manage the game. We need to think on our feet all the way through the game, roll with all the variables and find a way to win."
Meanwhile Johnson believes the battle up-front is where the quarter-final against France could be won or lost.
Despite conceding far too many penalties, Johnson feels his set-piece has been on top so far at the tournament.
However Johnson knows that his side face a completely different proposition against Les Bleus.
"I think we've got a very good scrum and a very clean scrum. We want to scrum it as do the French and it's a key battle for us," Johnson said.
"Last week, frankly, Scotland were happy to chance their arm with the referee and to get into a battle like that we weren't quite good enough to take it away from them and we ended up in a situation where it was going either way.
"We have to be that bit better in the scrums so there is no doubt who is under pressure. We got on top of Argentina's and Scotland's scrum and got the rewards for it."
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