Italy's finest actor brings fire, polish to any production.

Vittorio Gassman is stunning as 'Macbeth'; he'll tour US this fall Tempt a critic with superlatives and he squirms in his seat. He would far rather compress his enthusiasms into temperate terms.

For Vittorio Gassman, however, I fling my adjectives to the skies. Gassman himself is a superlative. Today's greatest Italian actor, he never touches a text without polishing it. And when Gassman plays Shakespeare - as he has been doing of late - his public revels in showers of gold.

Last season he gave us a profound ''Othello'' in which seething emotions were heightened as the actor rode them with a tight rein. This season he assails his spectators with a fierce, barbaric ''Macbeth'' - a stunning portrayal of ''vaunting ambition that o'erleaps itself.'' In Gassman's Macbeth there is the most desperate conflict of good and evil from first to last - an impersonation that expands into the racking fits of a tortured mind.

For his Lady Macbeth Gassman has chosen Anna Maria Guarnieri, a player who has run the gamut from Anne Frank to Lady Macbeth. She is now running to keep even step with Gassman, and with considerable success.

Both Mr. Gassman and Miss Guarnieri have taken a cue from Verdi's stage directions for his operatic ''Macbeth.'' Verdi advises the leading soprano to ''sing without singing'' - that is, to give her voice a touch of ugliness as befits the role. Miss Guarnieri's approach to the sleepwalking scene is no hollow-voiced soliloquy of one in a trance. Torn by quasi-psychotic passions, she caws like a crow, stamps like a wounded horse, waddles like an angry bear, striking horror into all who behold her.

Gassman, long noted for his vocal mastery, evokes the natural music and rhythm of the Italian text without declamation, without singing. All his words, motivated internally, ring true. The text is his own translation, a superb and faithful rendering of Shakespeare's verse. And for some bilingual ears modern Italian is more easily received than Elizabethan English.

Speaking of speaking, the voices of Gassman and Guarnieri are eerily blended into a kind of musique concrete - a taped sound track skillfully evolved by the computers of Giannandrea Gazzola. Thus the voices of the witches issue not from the throats of the players, but fly through the theater as borne on evil winds, fluttering in half-understood murmurs, ripples, gurgles. To his score Gazzola has added crickets, birds, wind, thunder, drums, and even bagpipes as Birnam Wood advances to Dunsinane.

Paolo Tommasi has designed a castle interior of crude woodwork, of stairs, screens, and planes that shift and blend as one violent scene follows fast on the heels of another. Splashes of color are added by Tommasi's costumes and most daringly by an outsize king's robe of royal red, spread casually over a flight of stairs like a blood-red river - a river in which Gassman wades with thick-soled boots.

It is clear that the varied elements of this magnificent production have been drawn into a unified whole by Gassman's masterminding. He is the director; he is the translator; it was he who filled the roles with excellent actors; and - most important of all - he has given us an unforgettable Macbeth.

Oddly, Shakespeare is a late-comer to Italian stages. Before 1856, when ''Othello'' was successfully staged in Milano, only two abortive attempts to present the Bard in Italy are on record: ''Hamlet'' was seen in Florence in 1791 and again in Bologna in 1795. Owing perhaps to inept actors, to mangled translations, and to a public steeped in non-English culture, two centuries slid by before Shakespeare captured the hearts of Italian theatergoers. Now he is Italy's favorite playwright, outdistancing even Italy's beloved son, Luigi Pirandello.

Gassman has a one-man show that recently completed engagements in Rome, Prato , and Pisa and is in Ravenna through next Sunday. It then plays in Naples March 20-31. During September and October he takes it to the Americas - Brazil, Uruguay, and Argentina; also to major cities in the United States, including New York and Los Angeles. Consult your local newspapers for dates and places.

In the one-man show, Gassman is presenting three monologues: Kafka's ''Report to the Academy,'' Pirandello's ''Man With the Flower in His Mouth,'' and a takeoff on Chekhov's discourse on the evils of tobacco, written by Luciano Coditnola and titled ''The Theater Is Not Good for You.'' He will give the Pirandello in Italian, but the two others will be given in English or in Spanish , according to the needs of his audiences.

Meanwhile, during April, he will be teaching large classes of young people at his own school, called La Bottega, in Florence.