Swiss harpist; Yankee ranconteur; Italian commedia fables; 'Giselle'; Spalding Gray and 'Blue Monster'

For an actor unshackled by fear, inprovisation presents unlimited possibilities for great wit, warmth, and eloquence springing off the cuff. It's a tremendous challenge, and if done well, is very rewarding for the audience.

Spalding Gray, an actor, philosopher, and sit-down comic, has mastered the art of improvisation in a quiet, low-key way. His autobiographical monologue, ''Travels Through New England,'' seen last week at the Brattle Theatre in Cambridge, spins out by itself, kept on track by only an occasional clue word.

This monologue (the 10th that he has performed in the United States and Europe since 1978), was in its early, raw stages. Gray tapes all his performances, and over time he will refine and hone his images. He didn't falter , however. Seated at a desk onstage, pointing occasionally at a colorful map of New England, Gray's delivery was more like a casual conversation. His only concession to acting technique was tiny sips of water, used in the same fashion as George Burns's cigar.

Gray was commissioned by the New England Foundation for the Arts to go from town to town interviewing people, and his monologue comes out of his experiences on the road. This loner with ''Venus' flytrap ears'' presents himself as a chronicler of mores and manners, small inanities and delights: He describes a freight train grinding to a halt so that its three engineers can go buy strawberry ice cream cones. Later he lands in Lawrence, Mass., on the day it is voted worst city in the United States and the residents go all out trying to convince him that it's not true.

This is not apple-pie America - the monologue is shot through with dispair and dislocation. Some of his anecdotes are tasteless and the faint smell of ridicule covers almost everybody, from mill hand to Yuppie. But one leaves impressed with his ability to capture his impressions and transform them into a coherent chronicle of our times.

Another show using improvisation doesn't work quite as well. ''Blue Monster, '' at the Nucleo Eclettico, is an 18th-century fable by Carlo Gozzi which, in traditional commedia dell'arte fashion, gives only a scenario. Each night, the actors make up new dialogue for this fable of a couple forced to change identities and fight monsters in order to be reunited.

The dialogue is only as good as the actors improvising it, and this troupe is a mixed bag. Steve Avesen, as Taer, holds his own. Ursula Drabik, as his wife, Dardane, flounders unhappily. The others have varying degrees of success with feeding the story line, some with dazzling wit, others with 20th-century jokes.

''Blue Monster'' is one of 10 fables Gozzi wrote in trying to revive a dying commedia dell'arte when scripted plays by Carlo Goldoni were starting to take over. Gozzi also wanted to attack the advancing materialism of the age. Director Grey Cattell Johnson keeps all this in mind, but it's hard for the audience to do so when faced with actors who are treading water, in terror trying to think of something else to say.