TRICKS OF THE TRADE -- At the National through October 26.

In this country, there is an aura of heroism about film or television stars who go on the stage, as if this were a personal sacrifice. (In England, where they do it all the time, it's assumed to be because they are actors by profession, and therefore want to work with live audiences.)

So good for George C. Scott and Trish Van Devere, who are playing in "Tricks of the Trade" at the National Theater in a pre-Broadway engagement. Only why did they bother?

The play, by Sidney Michaels, is not a bad little spy story about an aging agent now turned to playing games with the Agency, while conducting a cautious and brittle middle-aged love affair.But it would make a better movie. As a matter of fact, it has made a better movie. "Hopscotch," with Walter Matthau and Glenda Jackson, is more or less the same idea.

Among the attraction of a good spy movie like "Hopscotch" are terrific scenery, wild chase and a profusion of high-luxury dry goods that the spies treat with cool carelessness, thus providing one of the few shocking thrills available to consumers in an inflationary society.

On the stage, these things are not so easily available. "Tricks of the Trade" has a posh office for a set, its heroine wears nine smart dresses in two hours, and when there is supposed to be nasty weather, some movement on the skylight looks like rain. It even has one special effect, which is, of all things, a movie screen for instant replays of the stage action.

However, there are no palatial residences or breathtaking mountains, no quick tour of the major cities of Europe, no sports cars spinning over, and you can't see the labels on the costumes. And futhermore, just to get the effects that are achieved, the curtain is constantly going up and down, while canned music is directed to the audience so that it's like being on hold on the telephone. One can ony imagine what Van Devere's dresser is going through to get her changed so quickly so often -- and the trouble is, one does sit there and imagine it.

Movies are supposed to be more visually rewarding with theater better for words and ideas. And "Tricks of the Trade" does contain some clever lines and plot twists. It's just that they don't seem big enough to fill a stage.