Renovations to the oldest theatre in Britain, Bristol Old Vic, have unearthed new evidence about the rowdy nature of public entertainment in the 18th century.

The 246-year-old building, which in 2007 suffered a period of closure and a threat to funding, officially reopens this week after a year and a half's makeover. As well-loved by architectural historians as by British actors, it has given up some surprisingly subversive secrets.

The £12m restoration project, funded by public donations and £5.3m from the Arts Council, appears to have brought the venue closer to a rather disreputable past. It is clear now that middle-class members of the audience in the Georgian age once paid more money to stand and shout together in a "pit" at the foot of the stage than to sit in comfort in the gallery, while those in the best boxes expected to socialise with the actors during the intervals. The atmosphere was irreverent and closer to that of a speakeasy or nightclub, particularly since the covert theatre was run illegally without a royal patent for the first decade.

A key controversy of the refurbishment has centred on whether the seats on the sides of the stage should have been reinstated. Their position was a concern for artistic director Tom Morris, who is determined to get close to the intimacy of the original theatre and hopes to set up something of the authentic feisty atmosphere.

"This is the most beautiful theatre in the country and it is also the most loved by actors because of its shape. Now at last, much of its original, lost geometry has been exposed again," said Morris, who co-directed the hit War Horse while working at the National Theatre. "Perhaps we can reconnect with the audience here in the way that was intended, in a way that allows performers to relish the presence of the public."

Morris and his team reached a compromise by fitting seating that can be removed in the pit and at the side. Archives confirmed that the original stage projected out into the audience in what is known as "a thrust". Although it is the only remaining 18th-century horseshoe auditorium in the country, it also became clear that in 1888 builders took an axe to the front of the stage. This was well before another infamous re-modelling almost a century later when the building was effectively cut in half in order to fit in larger pieces of scenery.

Theatrical historian Mark Howell believes drawings show that James Saunders, the 18th-century carpenter who built the theatre for architect William Paty, intended spectators to sit in boxes on the side of the stage floor. The plans show the same dimensions as the Rose Theatre of 1587, used by Shakespeare, Howell argues. They also share the same number of boxes and the same size stage, yet these similarities have been ignored in the past simply because the theatre was in Bristol, not London.

"Standing on the restored thrust stage front at Bristol's Old Vic feels very like standing on the stage front at Wanamaker's Globe in London because spectators surround you on three sides," said Howell. "The energy from the stage front is extremely powerful. When actors stand there, it's like a spotlight falls on them, even when the stage lights are not on."

For Morris, the theatre stands a good chance of capitalising again on its unique proportions. He described it as the "freak survivor" of an era when the average theatrical building stood for only 17 years.

During the restoration work, graffiti by one of the original carpenters was found, along with a hidden doorway that led from a box to the backstage area. Builders also discovered a huge upturned funnel in the ceiling that drew hot air out above a central chandelier to act as a basic air-conditioning system, while a tarred gutter in the flies was once used for rolling cannon balls to simulate thunder.

"The history of this place is enormously inspirational, but it has only survived because it has kept working. The minute we become just a heritage site, we die," Morris said on the eve of the first preview of a production of John O'Keeffe's Wild Oats.