France did show their mettle with a quarter-final win over New Zealand last weekend.
But in Cardiff, they were the underdogs. On Saturday, the pressure will be intense in front of a home crowd - and Johnson believes France will crumble if England are still in the game with 20 minutes to go.
"We need to start the game very well to let the French team know it will not be easy for them and they will have to fight for 80 minutes for every little bit of ball, every inch of ground," said Johnson.
"If the French start well, the crowd are in the game - and if you are getting waves of French attacks it can be very, very difficult.
"But if England can get into the game early, like we did against Australia, the team will believe in themselves much more.
"If England are still there hanging on or, I hope, having the advantage around the hour point it becomes a great place for us to be.
"England are happy to be in a close game with 20 minutes to go. If you are the favoured team and it's close, you feel the pressure more.
"The French will be nervous. It is natural, because they are the home team and it's a semi-final. There is that expectation - so if England can get to the last quarter it is going to be extremely tough for France.
"Semi-finals at World Cups are very special matches. To get through is everything. To lose is terrible."
Johnson has no doubt that if the game is to go into extra-time, then England will prevail.
"If it goes to extra time I would like to see England squeeze the pressure - because France will be very, very nervous," Johnson added.
"The pressure will be intense on France - and if it goes to extra-time I think England will win."