The Right Size are back. In their last devised show, Do You Come Here Often?, two men got stuck in a bathroom for twenty-five years. This time two men fall down the back of the sofa. Down there a surreal Alice-world opens up, peopled by Mr Todd (Freddie Jones), Edna the cleaner (a sack doll), and a man wearing one red shoe who rides a bicycle (Chris Lamer).

Founder member Hamish McColl describes their return to devising, after touring Do You Come Here Often? and Brecht's Mr Puntila and His Man Matti, as: 'Joyous! Getting back to our addiction to the live laugh and working without anybody telling you what to do.' Following a stint writing a radio show and pilots for the BBC, he and Sean Foley are full of renewed zest for live theatre. And the success of their last two shows has given them some financial security and opened up possibilities of ‘more props, and a bigger set'.

Design is integral in Bewilderness. A classic 'real theatre' rake with trapdoors – and puppets (on strings) – offers huge potential for physical play, as well as opportunities to exploit visual illusion. However, such a sophisticated concept meant the set had to be built before rehearsals began in January. So creative decisions had to be made unusually early, including a zany film shot in Egypt in September. Nevertheless, The Right Size policy of ‘creating right to the last moment' and improvising on tour still applies: 'You wanna see what goes on underneath that set during the show!' quips Hamish.

They improvise around a script for the first fifty shows or so, only settling into a fixed script when they’ve tested it out with audiences. ‘The best thing is when you're playing with the audience and you get a better line,' says Hamish. Meeting after each show to decide which lines should stick, means that they continue building the script well into the tour. Like Theatre de Complicite, they prefer reviewers to write about the show once it's reached London. By then, it's settled into its groove.

The live-ness of performing is what turns The Right Size on. McColl and Foley brought their skills as live performers to Mr Puntila and His Man Matti, playing Puntila and Matti respectively in the acclaimed production directed by Kathryn Hunter and adapted by Lee Hall. ‘That was great fun,' they say. ‘The challenge was to put a bomb under it, because Brecht's reputation is so doughy.’ And although they both agree they’ll do another text play at some point, getting into the rehearsal room and charging around making things up after three years has been a tonic. More than that, their new show exploits theatre's live-ness as openly as pantomime. Interactive moments are in-built on a number of levels, not least in the Beckettian sense of the audience as 'tortured souls'.

Whatever happens on stage in this respect has emerged organically out of the rehearsal process. It's the idea which comes first. Going into rehearsal they only knew that they would meet Freddie Jones down the back of the sofa: 'We knew we wanted to do this show with him, because he has qualities which are perfect for this.' Their main inspiration is each other, and the seasoned team of Alice Power (design), Jos Houben (director) and Chris Larner (music). Alice is responsible for the enormous broom, a truly great prop. 'Wouldn't it be great if someone was in a broom costume?' Said Alice in a design meeting. ‘Yes. Hilarious. We'll do it,' agreed Hamish and Sean, without the faintest notion of how to fit it into the show. ‘But the image meant something in our world,' says Sean. ‘That's how we work. Our job is to make it all link.'

The links are risky at times. Yet in the fairytale world underneath the sofa, anything goes, and the giant broom co-exists happily with a rubber-glove fire, sack dolls and the bicycle. Freddie Jones, working with The Right Size for the first time, talks of being ‘confounded in a Hamley's of invention’, and the toy metaphor is apt. For Bewilderness conjures up a world of child-like inspiration, a roller-coaster of imaginative treats.

Yet this show has a darker side, a different tone to their earlier work. It shares the strange logic of a Grimm tale, whilst working on a contemporary level. According to Hamish: ‘If I were to draw the bathroom show, it would be a bright box coloured blue. Bewilderness has more shadows in the characters, emotions and stories.' Sean points out that in the end it's about two people who drive into a lamp-post and die – but it's a comedy. The discovery that their style can support pathos, that its essential lightness gives them the freedom to go further, is a sign that their work is maturing. So too is the tighter structure. Hamish acknowledges that the earlier shows, like many other devised pieces, were full of brio but not very well structured. Maybe working on TV scripts has helped. 'We've set a high standard for ourselves about how well written it is,' he says, ‘and we're constantly trying to rationalise the storylines.' For Sean, a belief that audiences love stories means the gags have to operate as part of the plot: 'You're building from a lot of angles: the props, the set, design and music – they're all in there together in the making process.’

If there's one maxim for The Right Size it's 'play it rather than say it'. Like many physical comedy troupes, they admire the stars of the silent screen with their physical skills acquired through vaudeville. 'When the physical cuts loose into the comedy and you see the freedom of the body, then that's just gorgeous,’ according to Hamish. They used Feldenkrais a lot in the early days, less so now. But Feldenkrais has given them a reservoir of physical information and they are able to create hilarious comic 'body-riffs', as memorable as those of the Marx Brothers.

Where they eschew technique is in their use of puppets. ‘Well, they're not really puppets. They're just sack people we throw around,’ points out Hamish. When one of these (Edna, the cleaner) needed some repair work, Alice suggested Sean could bring her back on the train. He was horrified. What if he had to buy her a ticket?