The author tells Tom Kington in Rome that the comedian, who finds himself kingmaker of the Italian government, is taking his cues from medieval comics who bedevilled the powerful

What makes Beppe Grillo tick? After a quarter of Italians voted for his brand of populist insurgency in last week's general election, it is a question preoccupying the country's political class and much of the eurozone. According to Italy's most distinguished playwright and prominent Grillo supporter, the answer is simple.

"Grillo is like a character in one of my plays," says Dario Fo, whose satires on medieval and modern life have seen him handed a Nobel prize and hounded off Italian stages in a career that has covered 50 years. "He is from that school of medieval minstrels who played with paradox and the absurd," adds Fo.

Fo, 86, is best known for his play Accidental Death of an Anarchist, inspired by the death of a man in police custody in 1969, and has long been a leftwing hero in Italy. He publicly backed Grillo this year, co-writing a book on the comedian's fledgling political movement and giving him a ringing endorsement at a packed rally in Milan's Piazza Duomo days before the election.

In return, Grillo, 64, suggested that Fo be nominated as the next president of Italy, an offer that the playwright turned down.

The high-profile backing contributed to a campaign that achieved an astonishing momentum. As a result of the 8.7 million votes Grillo received, his movement is now the biggest single party in the chamber of deputies, which makes him a kingmaker in a hung parliament.

After building a cult following through his blog, which denounced the austerity drive of the former prime minister, Mario Monti, and dubbed ex-president Silvio Berlusconi "a saliva salesman" and "the psycho-dwarf", Grillo's breakthrough before the election came when middle-class professionals started to see him as the best way to express their alienation from Italy's self-perpetuating political class.

Experts and analysts have been drumming up ideas about new political paradigms in Italy ever since. Journalists mobbed Grillo all last week for clues as to what comes next. His only response so far has been to refuse an offer from Italy's centre-left Democratic party to work together in parliament, using characteristically earthy language to describe the party's leader Pier Luigi Bersani as an "arse face". On Saturday he said he would accept a centre-left alliance with Berlusconi, only to add "they will never do it."

For Fo, the key to understanding Grillo is not in 21st-century Italy but in the 13th century, when storytellers – giullari – roamed Italy, entertaining crowds in piazzas with lewd and ancient tales interwoven with satirical attacks on local potentates.

"In English the equivalent word is 'juggler', but in Italy they juggled with words, irony and sarcasm," says Fo, who has attended Grillo's shows for years.

Grillo rose to fame mixing comedy routines with references to political scandals in the towns he was playing in, a straight lift from his medieval peers.

"He is from the tradition of the wise storyteller, one who knows how to use surreal fantasy, who can turn situations around, who has the right word for the right moment, who can transfix people when he speaks, even in the rain and the snow," explains Fo.

At one rain-soaked pre-election rally in Viterbo, in Lazio, central Italy, Grillo yelled: "Put down your umbrellas, I want to look you in the face." The crowd duly obeyed the comedian's demand.

Even the internet-based forums where Grillo's followers argue over policy have their roots in the Middle Ages, argues Fo. He says: "We had extremely democratic town councils in medieval Italy which knew the value of working together and every now and then, down the centuries, this spirit returns."

Grillo's focus on the web followed his ejection from Italian state TV in the 1980s after he made fun of corrupt Italian Socialist politicians, a few years before many of them were rounded up during Italy's Clean Hands probe.

His TV ban was part of a proud tradition, says Fo. "Nothing has changed since the Emperor Frederick II issued a decree in the 13th century against giullari who criticised power."

Fo himself was thrown off state TV in 1951 after he adapted biblical tales as political satire, the start of a series of run-ins with Italy's fascists, communists and the Vatican as his radical theatre group challenged taboos.

By 2004, Fo was being sued by an associate of Berlusconi after he staged a satire that poked fun at Berlusconi's small stature. "Every time you touch those who have power over the media, they seek to stop you," he says.

As a young man in Milan during the second world war, Fo helped his father – a resistance fighter – smuggle escaped British prisoners of war into Switzerland and his memories flooded back when he was invited on stage by Grillo at the Milan rally.

"The end of the war was the last time I saw that piazza filled with the same joy, with people changing their way of thinking about politics," he says.

Fo draws a parallel between Grillo's Five Star Movement's attack on Italy's privileged political class and the activists he worked with in the late 1960s. "Back then, people were also realising the importance of culture, of schools, and a generation of Italian singer-songwriters were giving voice to that."

The difference is that those artists never held the balance of power in Italy as Grillo does, with 162 deputies and senators under his movement's control in parliament. Now, after his election triumph, Grillo faces the challenges of real politics.

The first came last week when thousands of supporters urged him to form a functioning government with the centre-left leader, Bersani, who needs his backing in the senate to reach a majority.

"It is not easy, the Democratic party treated Grillo with disrespect, called him a fascist, a buffoon, but now they are offering their hand," says Fo, who is actively encouraging Grillo to negotiate, meaning that a playwright and a comic were making Italy's political headlines at the end of the week.

In Sicily, where the Democratic party runs the regional council but Grillo's movement is the biggest party, the two have formed a cagey alliance. "This is the model, it is working," explains Fo.

The real trap for Grillo, warns Fo, is being beguiled by flattery. Turning again to history, he cites Cola Di Rienzo, the charismatic son of a tavern owner in the 14th century who wooed Romans with his oratory and became the city's leader, setting his sights high and ousting corrupt noble families, only to see his support slip away before he was murdered by a mob as he sought to flee in disguise.

"I have seen the glowing press for Grillo and he must be careful not to fall for the adulation, it's a honey-like trap."