On Tuesday night, when Joe Biden and Donald Trump step on stage for their lead-off debate, they will take part in a ritual that since 1960 has been a feature of American presidential campaigns.

For voters, debates represent a rare moment when, in what amounts to a televised job interview, candidates for the highest office in the land stand side by side before the nation, with the citizens themselves in charge of the hiring.

The exercise can be both uncomfortable and revealing, as in any work interview. The way all work applicants do, debaters aspire to show their best representations of themselves.

But these auditions are held in front of tens of millions of people, unlike conventional job interviews, with the other candidate for the gig poised only a few feet away, ready to pounce. Anything can happen in such a combustible setting, without a safety net.

Many lawmakers would actually prefer not to have to participate, considering the dangers of discussion. But debates are no longer optional, especially at the presidential level. They have become an expectation which millions look forward to.

It is difficult to imagine a modern campaign without presidential debates for the American public, and for the country's political media.

There was no hint that these activities would take hold in 1960, when John F Kennedy and Richard Nixon inaugurated the tradition. In fact, the strength of these first debates and the unintended harm they caused to the candidacy of Nixon led many analysts to believe that such an experiment would never be replicated.

And they were almost right. The young president did not live to see a second presidential bid, even though Kennedy had verbally committed to debating his rival in 1964. His successor, Lyndon Johnson, did not want to debate, nor did Richard Nixon, the Republican candidate in both 1968 and 1972.

Nixon pointedly steered clear of challenging his opponents on a debate platform after having learned his lesson the hard way.

Only in 1976 did the tradition restart when underdog incumbent Gerald Ford challenged Jimmy Carter to a series of TV debates.

The major party candidates have come face-to - face before voters on live television in every American presidential election since. Three debates between presidential nominees and a single debate between vice presidential candidates are usually included in each cycle, although there have been as many as four (Kennedy-Nixon) and as little as one (Reagan-Carter).

Public reaction to TV debates is nothing short of immense and has been since the days of Kennedy-Nixon. In the United States, the Super Bowl is inevitably the most viewed TV show of every year, with ratings of upwards of 100 million people.

Yet the second-most-attended program is often a presidential debate in election years. The first debate between Hillary Clinton and Trump in 2016, for instance, drew a television audience of 84 million, with millions more watching through digital platforms. That year, only the Super Bowl attracted more viewers.

Vote scores are not the only indicator of the interest of the electorate in debates. In addition to being the second most-watched TV show of 2016, the same Clinton-Trump debate also broke records as Twitter's most-tweeted-about event to date.

Their living, unscripted nature is what makes discussions so interesting to viewers, and so intimidating to politicians. People watch a discussion understanding that the whole enterprise will go off the rails at any moment.

Presidential campaigns are concerned with exercising influence over communications, media coverage and how the electorate is engaged with candidates. Yet the opposite of a controlled scenario is portrayed by TV debates. They can neither foresee nor control what happens on the debate stage, no matter how hard the campaigns operate. Outside the limits of choreography, those 90 minutes of live broadcast unfold.

This is why, in the days and weeks leading up to the debate, campaigns work so hard to get their candidates ready. Although Trump is not supposed to train in any traditional way, a pre-debate model that has been in place for decades will be closely followed by the Biden team: a kind of 'boot camp' in which the candidate gets physically, mentally, and psychologically steeled for the fight.

In briefing books about the policy problems likely to be addressed, Biden will bury himself. He will familiarize himself with Trump's record in minute detail, particularly the more incendiary public statements and tweets. Biden would rehearse precise terms to be used in the debate, deliberately formulated in the press and on social media to draw attention.

He is going to go through a series of full-scale, real-time mock debates against a Trump stand-in. This mock Trump will survive withering attacks, then watch a video replay of the session as his coaches give their criticisms. Even the presence of Biden's cosmetics, his clothes and make-up and hair, will get a detailed peek under the lights.

With this intensive planning, we would expect debaters to be able to handle anything that could occur. But it takes a life of its own whenever a live debate comes on the air.

Nevertheless, despite their exposure as media mega-events, and their appeal to the public, debates seem to have a marginal influence on the results of elections. Presidential debates are more likely to reinforce established preferences than to change minds, especially in these periods of political polarisation.

The goal for candidates is to create excitement among their supporters, to use debates as a way to organize the vote.

Much as in daily job interviews, the position will eventually be offered to only one candidate. And, just as in the business world, the candidate who performs worse in the interview can still land the position, which is what happened in 2016 with Trump and Hillary Clinton.

As history shows, debates are many items in the final analysis-insightful, engaging, competitive and convincing. But rarely, on their own, do these televised work interviews decide who gets hired as US President.