For 10-year-old James the highlight of his tour is the flickering image of an overweight Roman beamed on to the wall of the circular bath. "That's really funny," says James as he watches the virtual Roman shed his towel and expose a round bottom. "It helps you imagine what it was like here."

The task of bringing one of northern Europe's most important Roman sites into the 21st century without turning it into a theme park is a delicate one. Today a five-year, £5.5m project to reinvigorate the Roman baths in Bath was showcased for the first time.

Shabby Roman and Victorian stonework has been rubbed, scraped and cleaned with lasers. The most photogenic area, the Great Bath, has been "de-cluttered" to make it easier to imagine what it would have looked like to a Roman 2,000 years ago.

New models, physical and virtual, help visitors understand what they are seeing, while there is a dizzying selection of audio guides in various languages, the English version being narrated by Bill Bryson.

Almost 900,000 people visit the baths every year, putting an estimated £92m into the economy. The council-run attraction made a profit of £3.3m last financial year – and so it was considered impossible to close the baths down entirely for restoration. Instead it was closed section by section.

Stephen Bird, head of heritage services at the local authority, said the work was crucial. "We had to up our game to retain our position as a leading visitor attraction. We needed to bring the baths into the 21st century but without changing it into a fairground ride."

The restoration may prove controversial for including a cast of characters aimed at bringing the baths to life. Some, like the chubby Roman, appear as projected images. Others are actors playing characters such as the Roman lady, Flavia Tiberina, and the stonemason, Sulinus, who mingle with visitors telling their story to anyone who cares to listen.

Bird insisted "integrity" was at the heart of the redevelopment, so the characters are almost all based on people with connections to the original baths. The actors have been given life stories, so a Roman soldier quizzed by a visitor will be able to give an idea of his life.

One of the first tasks of the restoration was to clear out centuries of clutter. Chunks of Roman stone that had been crammed into alcoves and nooks have been moved out. "We've tried to strip it back," Bird said. "You get the same view a Roman would have had for the first time in a few generations."

The idea has been not to over-interpret the attraction. "We want it to be immersive but not interactive," said the commercial manager, Patricia Dunlop. "The best interpretative tool is the visitor's imagination."

But those who liked the higgledy-piggledy and grimy nature of the baths need not fear. Some underground areas are as gloomy and dank as ever and the ankle-endangering Roman pavement around the Great Bath remains.

On a sunny day most visitors seemed impressed. "I loved talking to some of those characters," said Stevie Groom, from the US. "I've had my picture taken with a Roman – I'm happy."