Memento Muriel : SYMPOSIUM <i> By Muriel Spark (Houghton Mifflin: $18.95; 192 pp.)</i>

When she is at her best, Muriel Spark does not simply write; she performs. Her novels are in the tradition of the big English stage actors; the late Sir Ralph Richardson, in particular, for his comic deflection and sudden, uncanny strokes.

Despite Spark’s conspicuous use of the author’s omniscient voice, we and she sink into her books, as with any successful fiction. They are worlds; flippant, disconcerting and not un-tender. Yet we are always aware that the author is aware of us, cajoling or startling us and noticing our response; leading and misleading us along the fine line that runs between a magician’s fraudulence and her magic.

“Symposium,” like her last novel, “A Far Cry From Kensington,” shows Spark at a performing peak. Perhaps these books are narrower and a little more deliberately astonishing--and therefore, a little less remarkable--than her dazzling early cluster: “Memento Mori,” “The Prime of Miss Jean Brodie” and “The Girls of Slender Means.” But they are a late jump of pleasure out of the lesser books of recent years. Again, the theatrical note: It is more like actors than novelists to age through a long career into a new edge.

The setting of “Symposium,” appropriately to its title, is an elegant and carefully orchestrated London dinner party. Two other literary evening gatherings are appropriately noted in an epigraph. From Plato’s “Symposium” comes the line about the close connections between the genius of tragedy and the genius of comedy. From Lucian’s lesser-known work of the same name, we read of a dinner that ended in a bloody brawl.

Tragicomedy, comic tragedy and a whole entanglement of brawls, including several murders, wriggle and splash beneath the crystalline surface of the dinner given by Hurley, a sardonic American painter, and Chris, “the grand-looking and rich” Australian widow he lives with in “a union of great convenience and contentment.”

The guest list is as carefully and contentedly considered as the menu. There is pheasant, with fish mousse to start. The fish may be salmon, but we know right off that we are to regard it as red herring. Spark’s is a comedy of mysteries, as well as of manners and morals; but she is perfectly up-front about them. She fools us while she tells us that she will; we are fooled, are given most of the answer at the same time, and we go on being fooled.

At one end of the table, Hurley is sympathizing with the distress of the 22-year-old Helen, third wife of the intolerable 50-year-old Lord Suzy. Hurley recommends the example of St. Uncumber, a medieval virgin who got rid of a suitor by growing a beard. At the other end, Lord Suzy is intolerably barking out “Rape!” as he tells over and over of the burglars who took his silver and urinated on his walls.

Ernst, a suave businessman, wishes Lord Suzy would change the subject. “Since dry champagne is being served in tubular glasses, he feels the details of Brian Suzy’s robbery are entirely out of place.” At the same time, Ernst is stroking the wrist of Luke, a handsome American graduate student hired as a temporary waiter. Both Ernst and his wife, who also fancies Luke, wonder about the young man’s new gold wristwatch; each suspects the other of giving it to him.

There is the steady patter of malicious gossip, dampened by the moistly charitable interjections of Margaret, who is there with William, her new husband. William is dim, plump and hopelessly in love; he is also the son and heir of Hilda, a rich and powerful Australian newspaper magnate. Hilda is expected to show up later for coffee. In fact, Spark tells us, she is being murdered.

Naturally, we are curious, and it is not long before our curiosity zeroes in on Margaret, whose history repeatedly leads us away from the dinner table and back again. We dislike her from the start. She wears green velvet pre-Raphaelite gowns to set off her red hair; her teeth protrude, and her niceness--she keeps stressing the need to think of others--is an affront to humanity.

Certainly, it is an affront to the dinner party. And pretty soon, we realize that several of the diners know worse things about her. She had ensnared William with cold military tactics, for one thing; clearly, she was after his mother’s money. And she had figured in press reports about the unsolved murder of her grandmother several years earlier.

Who, at present, is murdering Hilda is one of the mysteries in “Symposium.” But it is not the major one; Spark helpfully lets us know the answer before we are very far along. The real mystery is just who Margaret is.

She, it turns out, is one of Spark’s sublimely mixed-up characters--loathsome and appealing, possibly mad and possibly an innocent driven to strong action by a mad world. In this case, the world consists of her peculiar Scottish parents and her totally insane Uncle Magnus. Magnus spends weekdays in an asylum, and comes home weekends, dressed in purple shirts and green ties, to act as the family adviser.

Margaret, we learn, has been surrounded by mysterious deaths ever since she and a schoolmate went swimming and the schoolmate drowned. A tea-date with a teacher ended in the teacher’s permanent disappearance. The grandmother was killed by an escaped fellow-inmate of Magnus’, but it took place right after Margaret had instructed the old lady to change her will in favor of Margaret’s father. And when she takes refuge in a convent, one of the nuns is strangled.

All these things curl into the dinner party, like smoke from under the doors of an adjoining room on fire. Nothing is entirely what it seems, nor is it entirely other than what it seems. We know pretty much what happened; we don’t know quite what it means. The same is true at the end, when Hilda’s death is announced.

It is a grand performance, hilarious, mordant and troubling. Literature abounds in characters who are admired for goodness, know themselves to be wicked, and struggle to deserve their reputation. Margaret is known to be wicked, knows herself to be innocent--more or less--and likewise and oppositely struggles to deserve her reputation.

Spark is more than a superb entertainer. Take one central, and richly unsettling point: The worse we know about Margaret, the better we like her.