Dinah Casson and Roger Mann’s exacting and insightful approach to exhibition design has put them in the forefront of a complex discipline.

Aidan Walker talked to them about the V&A’s British Galleries, Great Expectations in New York, and objects in and out of context portrait James Harris Dinah Casson is in full flow, explaining the subtleties of a designer’s relationship with the curator and the content in museums, expounding the crucial importance of story as a carrier of meaning, and reiterating the central position of the visitor in modern museums. “Museums realise now that it’s about visitors. It’s a quantum shift from the old way of putting objects in glass cases with little bits of paper in front of them. The Science Museum has been ahead of the V&A on that.”

The differences between the work on the Science Museum’s Wellcome Wing, for which Casson and her partner Roger Mann received a quiverful of awards last year, and the soon-to-open British Galleries in the V&A – 400 years of decorative art, 2,900-odd objects spread over 3,200 sq m of exhibition space in 15 galleries – are legion. But, curiously, when it comes to discussion of the Casson Mann methods, it is the similarities that keep rising to the surface.

The telling of stories is a major theme; the role of objects is another ruling factor. In the Wellcome, explains Mann, who led that project, the “brief was not about objects but about ideas”. The V&A, adds Casson – her job – is all about objects. “There were no objects to start with in the Science Museum job,” continues Mann. “The objects are not in there to support the story – the gallery itself is part of the story. In the V&A, the objects are the story.”

As the conversation continues, their rigorous method and persistence of intellectual inquiry become apparent. How will we attract and keep visitors? How will we tell such a complex story? Will the design overwhelm or underpin the experience? Should objects be placed in context if it means creating that artificially? That is, should we cheat, build a roomset with original elements, but copy and create others? Are we supposed to be showing people what it looks like behind the scenes?

There is no doubt that Casson Mann has become the leading museum and exhibition designer in the country, setting the agenda and responding to it at the same time. Of course this pre-eminence, gained through reinvigorating and partially reinventing two of the country’s, if not the world’s, most venerated cultural institutions, has not come overnight.

The Wellcome Wing took three years, the V&A five. These long gestation periods themselves are indicators of the mould into which Casson Mann has – willingly – been cast. You would be hard pressed to find another consultancy so committed to mining the depths of meaning in the work, to search for multiple and subtle ways to communicate complex ideas, to “help people” – in Casson’s words – “engage with difficult objects”, even before drawing starts. “You need what I call ‘oven time’. You have to let it cook, get a deep understanding of the task. It was three years before I put anything on paper.”

The process is especially long when the stock in hand is the cultural heritage of a nation and the client is as labyrinthine and entrenched as the V&A. This is not to say it is unimaginative or cowardly – London’s most daring building for decades, if not ever, is rising from the ground on Exhibition Road – but the members of such an institution see themselves as guardians, and the weight of tradition is indeed heavy. “The V&A is hugely political,” says Mann, “with curators looking after their own patches.”

Which leads to the knottiest subject of all in the search for the key to Casson Mann’s success and to its method. How do the designer and curator relate? In the case of the V&A, the client structure was multilayered but logical – a pyramid headed by the chief curator of furniture, Christopher Wilk. Others dealing with, say, education, textiles or ceramics worked to him.

Then there were outside consultants, among them historian John Cornforth and traditionalist interior designer David Mlinaric, whose approach, it is apparent from Casson and Mann’s polite skirting of the subject, significantly differed from their own.

The Science Museum project, says Mann, “was not collection based. The content will date; the remit was to deal with contemporary science, not history. The galleries are about the future, bio medicine, digital technology and so on: some of the stories to tell were out of date by the time they were on gallery. The Wellcome galleries are very much about Casson Mann as a practice, working with one client representative, Heather Mayfield. The design of the gallery itself is part of the illustration of the story.”

“At the V&A,” continues Casson, “the objects are the story. No big design statement was allowed. It had to be understated. The objects are very powerful, so there is no Casson Mann; the design supports the objects.”

It is clear that the practice, led by its articulate and thoughtful principals, was never in any doubt about who was the curator. Yet the task of extracting or communicating meaning is very much theirs, and a topic at which their two pairs of eyes shine. This is where the subject of “layering” comes up. The objects in the 15 British Galleries are carefully grouped to support the exposition of four themes: “Who led taste?” deals with the changing social and political sources of influence. “Style” is treated chronologically.

“Technical innovation” exposes ways of making or developments in objects themselves – for example, once you have books you need bookcases.

“Fashionable living” is the consumer story, what it felt like to be disseminators of goods and habits such as tea drinking.

This “layering” leads to different ways of understanding and new forms of meaning, maintains Casson. “Do we impose meaning? No, it needs a light touch, even if it’s serious underneath.”

The debate about the discovery and attempt to express new meaning for the objects in the museum leads to another set of issues. New air conditioning has made it possible for textiles and other fragile items to be on show. Also, arrivals such as an Adam ceiling from the Adelphi have led to the formation of a Casson Mann policy of dealing honestly with historical environments, and making objects out of things that that were not originally objects. “The Adam ceiling,” Casson says, “is hung without walls, so you look on it as an object. You can’t see it as anything else, it’s not in a roomset. You can see how it’s made.”

There was also debate over showing the back of the panelling for the gilded music room from Norfolk House; Mlinaric’s team has regilded the “for show” side, and Casson Mann’s attempts to keep the shoddy-looking back view, with its roughly caulked seams and coarse joinery, visible have been whittled down to a humble square metre or so. Still, it is there.

“Reconstructions and roomsets are about history,” says Casson, “not decorative arts. We’re not supposed to be making people feel ‘what it was really like’ – there are plenty of beautifully preserved country houses within a few miles of here. Museums forget the intelligence of their visitors.”

Thus Casson Mann was involved in what is essentially a curatorial debate. By and large it has won, although there are complete roomsets here and there in side galleries. But taking the “orphanage” line with objects, as Casson calls it – showing them in their own right without faked context – leads one to the problem of orientation. How are visitors to make sense of what they see? And how to deal with the serene and familiar fabric of Aston Webb’s building itself?

This is how the “floating wall” device, which is the basic building block of the exhibition, developed. If objects are orphans, you need something behind them to tie things together – in this case colour. Chronological context is given by use of meticulously researched and applied colour, used on panels behind groups of objects. Behind the panels are a range of larger, more coherent and more neutrally coloured panels intended to show that the whole display is an installation.

Why objects look the way they do and how they are made is, says Mann, shaped by nothing other than dialogue with the environment. “Our job,” says Casson, “is to identify and clarify the dialogue between creator and environment that resulted in that object.”

All this dialogue about dialogue leads – late in the day – to Great Expectations, the Design Council travelling exhibition about British design innovation, opening in the Vanderbilt Room of New York’s Grand Central Station on 14 October. Casson Mann put together an entire team – curators Helen Jones and Libby Sellers of Restructure, project manager Richard Greenwood and graphics consultancy Kerr Noble – and pitched for the job nine strong.

It must be said that the idea was already there. “We knew the venue when we pitched,” says Mann. “It’s a long, narrow and very high room, an amazing space, in Grand Central Station. People hurry through it all the time. The Design Council wanted something that would stop them in their tracks. We looked at the space and thought it would be great for a banquet – where you get conversation, dialogue. So the idea developed of inviting people to sit at a huge long table and engage.” The basic form of the show is an illuminated table 50m long, holding the objects and their animated explanations.

“Casson Mann is an exhibition communication expert,” says Jones. “You don’t have to have all the skills in-house. It’s good at finding collaborators, at dealing with the dialogue between curator and designer. To be an exhibition designer is not the same as having specialist skills in research or content provision. Its skill is selecting the right people.” One of its skills, certainly. What makes Dinah Casson and Roger Mann – and their colleagues in the still small studio – unique is an ability to combine the “big idea” with painstaking and intellectual rigour. They will test and harry that idea until it breaks, or until they are sure it works.

There is depth, there is lightness, there is seriousness. Above all, there is wit, which in the centuries dealt with by the V&A’s British Galleries meant just “clever”. Now it means funny and inspirational, insightful and irreverent as well.