Polls suggested the centrist Macron had a 20-point lead on his far-right rival but the viewers want to see who is best up to the task of leading the country.
Some 17.8 million people watched the last debate in 2007 between Socialist François Hollande and conservative Nicolas Sarkozy. This time the figures are expected to be even higher, not least because there is a far-right candidate in the runoff.
The last time that happened was in 2002 with National Front founder Jean-Marie Le Pen. His mainstream-right opponent Jacques Chirac refused to debate with him out of fear of "normalising hate and intolerance".
But 15 years later the situation is different. Le Pen scored 21.3 percent with 7.68 million votes in the 23 April first round and the National Front is now a real force in French politics.
The two finalists crossed swords in debates before the first round, with Le Pen memorably accusing Macron of waffling for seven minutes and saying nothing. Macron meanwhile said she was transforming France’s millions of Muslims into “enemies of the republic”.
The two candidates have promised not to spare one another in tonight’s face off.
On Tuesday Macron told BFM television he wanted “to go head to head, to get to the bottom of the issues, to show that [Le Pen’s ideas to tackle the country’s deep economic and social problems] are false solutions”.
Le Pen has dismissed Macron as the face of finance and an advocate of unbridled globalisation and immigration, and a proxy for the unpopular Socialist President François Hollande.
"If he finds himself in difficulty, he can always ask François Hollande to come and hold his hand. I won't complain," Le Pen tweeted Tuesday.
Battle for presidential image
The televised debate is a tradition in France, dating back to 1974.
“Everybody watches it, it’s a bit like in a corrida [bullfight],” says political scientist Isabelle Veyrat-Masson from the French CNRS research institute. “Perhaps there will be a fight to the death, everyone is waiting for something special.”
But, given that the candidates are both relative newcomers to this kind of exercise, Veyrat-Masson says viewers might not get their fix.
“It’s the first time we have a candidate from the far right and a very new, very young candidate, so that makes the debate very exciting. But I’m afraid it will be very classical because the two have to show they’re capable of being president, that they’re not aggressive and that they’re capable of keeping their calm.”
Staying calm and yet making an impression. No mean feat.
“The final debate in France is always about image, it’s about the personalities of the candidates,” says Philippe Moreau Chevrolet, a communications specialist with MCBG Conseil.
“The wittiest guy or girl, the most authoritative guy, the person who will be larger than life in the room and will dominate the other is the one who is winning.”
The person who "wins” the debate won’t necessarily win the election, though.
“The debate doesn’t usually change the result and by now we are fairly sure that Emmanuel Macron should win the election,” Chevrolet continues. “But still the debate is important this year because a lot of voters are undecided”.
So how can we expect the candidates to perform?
“Marine Le Pen could really make a good impression,” says Chevrolet, “if she manages to appear pleasant enough, appeasing enough, and not at all like her father Jean-Marie Le Pen was.”
Le Pen has done her level best to distance herself from the veteran racist, anti-Semitic firebrand Jean-Marie, even removing the National Front logo from her latest campaign poster. Chevrolet expects her to further push the new, softer image she has been cultivating of late.
“Marine Le Pen is trying to play both the seducing woman card and the motherly card. She’s been treating Emmanuel Macron during the previous debate very much like a small child. Smiling at him, not taking him seriously but not being overly aggressive towards him. It’s a very clever strategy.”
Le Pen is only nine years Macron’s senior but she was born into politics, growing up in an albeit marginalised party. Le Pen has tried to convey the message Macron is an impostor and that only she has the credentials to run a country.
So the challenge for the young, former banker is to counter her punches, yet stay cool and presidential.
“Macron needs to be stronger, needs to appear more authoritative, to feel like a ruler, like the head of state that he aspires to be, and French voters are not convinced that he’s able to do the job yet,” says Chevrolet. “He has to fill the very large shoes that General de Gaulle has left behind him. So it’s not easy for him to do that.”
One of Macron’s strong points is that “he doesn’t let things go” says Veyrat-Masson citing the way he dealt with rumours of his homosexuality head-on and dared to face hostile workers at the Whirlpool factory in his home town of Amiens.
In preparation for the debate, candidates generally wade through previous ones. Chances are they will both have poured over the 2007 edition between Nicolas Sarkozy and Socialist Ségolène Royale in which Sarkozy accused his rival of losing control of herself.
“I haven’t lost my cool, I’m angry,” Royale fired back.
“Well I wouldn’t want to meet you when you’ve lost your cool then,” Sarkozy quipped.
“It was the only time when we saw someone becoming a bit furious [during a debate]”, says Veyrat-Masson, “because she had to prove that despite being a woman she was strong, firm and able to stand up to a very male Nicolas Sarkozy.”
Reading the signs
Someone who will be watching every move, not just listening to every word, is Stephen Bunard.
He is a synergologist, or specialist in reading non-verbal communication.
Bunard says it no surprise Le Pen and Macron have made it through to the run-off.
“They have the most eloquent body language, like Jean-Luc Mélenchon, who also had a good score,” says Bunard.
Interestingly enough, despite being poles apart on policy, they do have something in common.
“They both have unconscious codes of seduction,” Bunard explains. “Marine Le Pen has it through the way she clasps her hands together, fists held high which shows her dominance.”
He has also identified the way she raises her eyebrows a lot to draw attention to the important things in her speech and “often slants her head to the left, in an attempt to connect with the person she’s talking to”.
Macron uses similar gestures “but in a slightly less controlled way”, says Bunard. But in both cases the candidates reveal their true colours in moments of stress and tension.
“We’ll be looking for signs that show the person is living through a stressful moment, when the person is, intellectually, not fully connected to what he or she is saying.”
In Macron’s case this is likely to be on presidential powers like national security and defence.
“He’s clearly not at ease and still hasn’t quite resolved in his mind how to reply,” says Bunard. “I’ve noticed he raises his left eyebrow and tilts his head to the right as a sign of wariness.”
The other subject the centriest candidte appears uncomfortable with is how he will form a government should he be elected.
“It continues to be a problem for him because we can see he often scratches his nose. That’s related to concern over the image he’s putting across.”
As for Le Pen, she tends to “take on a slightly awkward, artificial smile” when she’s uncomfortable, most often on economic questions according to Bunard, “precisely where Macron is most at ease.”
Whether or not these kind of gestures are likely to impact public opinion is another question. But one thing is clear, communicating in a non-verbal way does at least keep viewers watching.
“Macron resembles Nicolas Sarkozy [in that way], they have a similar moral fibre,” Bunard concludes. “He uses more gestures than Le Pen so he’s perceived, unconsciously, as more authentic, just like Sarkozy was. Not everyone liked Sarkozy but they all watched him.”
That could be good news for Macron. He’ll have the last word in tonight’s debate, while Le Pen will open the match.
To read our French presidential election 2017 coverage click here