We often assume that art is about freedom, breaking conventions, opening things up. But most artists would agree that really it's much more often about constraints and limitations. A photographer might choose to work in black and white, or a dramatist write for radio, or a musician compose for one instrument and in one key, as a way of testing what can be done with limited resources.

In Off & On, which had its world premier at the Playhouse in Fredericton Thursday night, David Hughes sacrifices what we often think of as the theatre's central resource, language. In the whole two acts -- an hour and a half of mostly entertaining and engrossing theatre -- there's not a single word spoken.

We know that David Hughes can use language: we've seen him often, doing fine work at Theatre New Brunswick in plays like The Gin Game and The Importance of Being Earnest, where language is the essence of the experience. But in Off & On, the one-man show Hughes has devised (we can't say "wrote"), everything -- every nuance, every emotion, every surprise -- has to be conveyed by the actor's body, and by the music, lighting, sound and props.

This is a challenge -- to the actor, and to the supporting company. And by and large, everyone rises to it. Off & On is an hour and a half of engrossing and amusing, if not entirely satisfying, theatre.

Much of its success is due to David Hughes' astonishing talent for mime. Watching his work in other productions, we've seen that he has considerable physical skills, but I hadn't been prepared for some of the brilliant mime he pulls off in this show. From the first entrance, when his bumbling about the stage he's been hired to clean, startling himself with the lights and playing with the sound effects machinery left from some previous production, he held the audience's attention firmly. At a number of points I thought of Charlie Chaplin, and occasionally of Rowan Atkinson's more grotesque "Mr. Bean," but Hughes' character is all his own.

The premise of the play -- which is really only clear from the program notes -- is that he's been hired to clean up an abandoned theatre. As he works he gets distracted by various props and objects, and finds himself "on" -- teaching a chair to dance, or enacting a "dialogue" between a painted tree and an axe-wielding lumberjack, or struggling with his right hand, which has become a character in its own right and simply doesn't want to play the violin he finds.

The separate "bits" -- there are about 18 listed in the program -- vary in quality, but none are trivial and some are brilliant. I was particularly taken with his being dragged about the stage by a butterfly and with "The Neighbours," in which Hughes plays two characters simultaneously, using only a housefront with two doors in it. He distinguishes the characters with his face, with his hands, and with his posture, and does it so thoroughly and consistently that we hardly need the costume changes -- one has a green bow tie and the other a baseball cap -- to keep them straight as they chase each other in and out of the doors.

One of the ways the characters are kept straight for us is that Neil Bartram, the composer and offstage pianist, provides unobtrusive but extremely effective musical cues, in this scene and elsewhere. The music, throughout, is a powerful support for Hughes, often helping the audience understand a not-quite-clear routine.

Even more impressive than the music, however, is the use David Campbell makes of the Playhouse sound system. I've never heard it used to better effect, from the train we could almost see thunder across the stage, to the invisible squirrel who quite obviously entered stage left, crossed and ran up the branch of Hughes's "tree." Timothy Gorman's lighting design is almost as effective; I especially liked the way the lighting signalled a dream sequence.

Still, the show has its problems. The most important is what I can only call lack of structure. It's not only not very clear from the action on stage what the character's intentions are; there's not much, if any, sense of progression between the often rather arbitrarily separated "bits." This means that it's rather more like a magic show than a piece of theatre: some tricks are wonderful, some less so, but they don't add up to much, and they don't gather any momentum.

After you've laughed at the brilliant "The Chair," in which Hughes first becomes a chair, then teaches it to walk, then dance, then dances with it, you then may not laugh so much at "The Tree," or see why it should come next, or come at all.

And that's another problem with the show: it just seems slightly too big for its material. To fill the Playhouse, to give us an evening's theatre, with an intermission, it has to be an hour and a half long, but it felt as though Hughes really has about 45 minutes' worth of material. Many of the bits -- even some of the most brilliant ones, like "Neighbours" -- seem, even while you're enjoying them, to go on just a bit too long. And since there's no logic by which bits are included, you can't help asking which ones might have been cut if the show had been designed as a one-acter.

And finally, there were a number of times Wednesday night when it seemed to me either Hughes' timing wasn't quite on (perhaps this will have tightened up for the official opening on Thursday) or the bit hadn't been quite worked out -- for instance, there is a long wrestling match with a push broom in which it was never clear -- to me, at least -- what the premise was.

Still, in what was a pretty unresponsive preview night house, there was enough laughter and enough warmth of response to make be believe Hughes has a potential winner in this show. Take kids to see it; they'll rarely have a better chance to see what real theatre can be -- and if the young lady on her dad's lap a couple of rows behind me is any indication, they may think it's just as funny as I did.