When Barack Obama steps up to the podium to deliver his inaugural address, one man standing anonymously in the crowd will be paying especially close attention. With his cropped hair, five o'clock shadow and boyish face, he might look out of place among the dignitaries, though as co-author of the speech this man has more claim than most to be a witness to this moment of history.

Jon Favreau, 27, is, as Obama himself puts it, the president's mind reader. He is one of the youngest chief speechwriters on record in the White House, and, despite such youth, was at the centre of discussions of the content of today's speech, one which has so much riding on it.

For a politician whose rise to prominence was largely built upon his powers as an orator, Obama is well versed in the arts of speech-making. But today's effort will tower over all previous ones.

It is not just that Obama has set an extremely high bar by invoking the inaugural speeches of Abraham Lincoln as his inspiration - admitting to feeling "intimidated" when he read them. It is also that, as he begins his term with the US in an economic crisis and two wars, he knows he needs to kick start his presidency with a soaring rhetoric that both moves and motivates the American people.

The tone of the speech could be decisive in determining how the public responds to his first 100 days, as Franklin Roosevelt's famous line "the only thing we have to fear is fear itself" helped to determine his.

Obama aides have let it be known that a key theme will be restoring responsibility - both in terms of accountability in Washington and the responsibility of ordinary people to get involved. Rahm Emanuel, Obama's chief of staff, talks of a "culture of responsibility" that would "not just be asked of the American people; its leaders must also lead by example."

In composing the high notes of the speech, Obama has leant on Favreau, whom he discovered almost by chance four years ago when the younger man was working on John Kerry's failed presidential bid. "Favs" has since studied Obama's speech patterns and cadences with the intensity of a stalker. He memorised the 2004 speech to the Democratic national convention which first brought Obama into the limelight. He is said to carry Obama's autobiography, Dreams From My Father, wherever he goes. As a result, last November when Favreau sat down to write the first draft of the inaugural address, he could conjure up his master's voice as if an accomplished impersonator.

That skill had been put to almost daily use in the 18 months of brutal campaigning on the presidential trail. Favreau would be up most nights until 3am, honing the next day's stump speeches in a caffeine haze of espressos and Red Bull energy drinks, taking breaks to play the video game Rock Band. He coined a phrase for this late-night deadline surfing: "crashing".

He crashed his way through all Obama's most memorable speeches. He wrote the draft of one that helped to turn Iowa for Obama while closeted in a coffee shop in Des Moines. For the presidential election, he wrote two speeches: one for a victory, one for defeat. When the result came through, he emailed his best friend: "Dude, we won. Oh my God."

The tension between such youthful outbursts and his onerous role has sometimes cost the 27-year-old. In December, pictures of him and a friend mocking a cardboard cut-out of Hillary Clinton at a party, Favreau's hand on her breast, were posted on Facebook to his huge embarrassment.

Obama is an accomplished writer in his own right, and the process of drafting with his mind reader is collaborative. The inaugural speech has shuttled between them four or five times, following an initial hour-long meeting in which the president-elect spoke about his vision for the address, and Favreau took notes on his computer.

Favreau then went away and spent weeks on research. His team interviewed historians and speech writers, studied periods of crisis, and listened to past inaugural orations. When ready, he took up residence in Starbucks in Washington and wrote the first draft. The end result will be uttered on the steps of the Capitol.

Obama's mind reader has crashed his way through yet another deadline.

• This article was amended on Friday 23 January 2009. Jon Favreau, age 27, is not the youngest chief speechwriter in the White House. He shares that honour with James Fallows, who served as Jimmy Carter's speechwriter at the same age. This has been corrected.