WHAT, I asked myself, was a nice quiet academic doing getting involved "as an expert witness in the matter of the Commonwealth of Massachusetts vs. Newsconcorp, Inc., Saxon theater of Boston, and Penthouse International, Ltd. [the "Caligula" movie case] in Boston, Mass.?

That's how the legalese ran in the letter addressed to me on July 18 by the New York law firm of Grutman & Schafrann.

I had been recommended by an expert in erotic cinema, who called from California asking whether I would consider testifying for Bob Guccione's Penthouse Films production. He knew that I had given courses in fim -- both at Northwestern University and later at Maryland -- and that I have been an occasional film critic for the London Times Literary Supplement.

I had not seen "Caligula," although, like most movie buffs, I had heard of it. It wasn't the sort of exercise I would normally go out of my way to catch. But I take a very serious view of the Constitution and the rights guaranteed by the First Amendment, so of course my curiosity was piqued.

Moreover, the eight-day trial in Boston Municipal Court differed in certain key respects from the garden-variety obscenity prosecutions. The usual hard-core movie consists of little more than permutations on the postures and mechanics of copulation. But "Caligula" had a cast that included an illustrious theatrical knight. The movie claimed to be "adapted from an original screenplay by Gore Vidal." Danilo Dorati, the art director for sets and costumes, had three Academy Awards to his name. The film had set the producers back a cool $17,500,000. And the state was calling it obscene.

So that Saturday my wife and I attended an early showing at the Georgetown theater. I like to think I had no preconceived ideas about the film, but I'm no Guccione fan. Penthouse magazine features Penthouse Pets -- an odious sexist term reducing women to more or less the same status as cocker spaniels -- in poses no doubt highly instructive to first-year medical students.

The first thing I discovered at the theater was that the price of admission was $6. In New York, I learned, they were getting $7.50. Whatever the merits of "Caligula," the tab was certainly obscene. (But then, expert witnesses would receive, apart from expenses, a fee for their time. I had asked for $1,000, and in the end a check for a thou awaited me in Boston.)

"Caligula" didn't strike me as a porno film as that term is usually understood. True, there were a couple of explicit sex passages. One was a lesbian encounter performed by evidently house-broken Pets stimulated by the voyeuristic opportunities of Caligula's court. The other was an episode of heterosexual oral sex, complete to the final spasm. What most struck me as odd, however, was that the male sex organ was in effect completely disembodied, the human being to whom it belonged not otherwise being glimpsed. The performer had no face or personality.

All the fuss up in Boston was being made over these two passages, occupying maybe 10 minutes in a film that ran for over 2 1/2 hours. In crassly obscene films, sexual episode usually succeeds sexual episode, everything building up to the final big group effort. Nobody in Boston, so far as I could tell, seemed much concerned about the film's cruelty. Featured was Caligula's huge rotating killing machine, a monstrous Roman Cuisinart that publicly lopped off the head of Macro, the unfortunate commander of Caligula's Praetorian guard. Then there was the torture of Proculus, who had never done Caligula any harm -- "Why me?," he plaintively begs his emperor during the hideously drawn-out ordeal.

Still, the cruelties were part of a world that countenanced such excesses, and I doubt that anything depicted in the film would have raised the eyebrow of old Suetonius, who chronicled Caligula's reign. And, of course, the emperor is mad, a fact of which we never lose sight. Caligula is in fact quite devoted to his horse, Incitatutus, which at one point is taken to bed by the emperor, and afterward duly proclaimed a senator of Rome.

On the flight to Boston I read a document, supplied by counsel, on "Use of Expert Testimony in Obscenity Cases." The pasage that especially grabbed me had to do with "the necessity of challenging, if at all possible, the expert's credentials as they relate to the subject of testimony. A national name or an impressive educational and professional background may be effectively dealt with by showing, on cross-examination, that the expertise, however impressive, does not relate to the matter at hand." How would I fare when the prosecution had its inning? All those movie courtroom melodramas were rattling around in my mind.

Late that night at my hotel I had a session with William Homans, the Boston co-counsel sharing the defense with Roy Grutman's New York firm. He sounded me out about my educational background, professional experience, honors and the like, and about my impressions of the film. Never did he try to press a point or attempt to influence my opinion, although naturally he needed to know enough about my views to formulate his questions.

In court the next morning there were some surprises. I had been looking forward to high drama in a crowded courtroom, but the room was almost deserted: fno reporters, so far as I could tell, and hardly any spectators. Lots of lawyers, though: three connected to the film's defense, and another representing the hapless management of the theater, which had temporarily substituted a Walt Disney feature. Presiding was the chief judge of the court, Harry Elam, a black man of distinguished, reflective bearing, in his 60s I guessed, with hair silvery at the temples. The personable assistant D.A. was Timothy O'Neill. Roy Grutman I had met the night before. He was a natty dresser, sporting a straw boater selected from his wardrobe of hats. Grutman's eye twinkled as he held forth in orotund tones. Like me he hailed from the Bronx. He made a nice contrast with the proper Bostonian Homans. By mutual consent both sides in the case had waived their right to a jury trial.

At issue was whether "Caligula" had serious literary, artistic, political or scientific value: the so-called "Miller test." A parade of professional authorities on literature, the classics, philosophy, linguistics and film testified. Two psychiatrists for the prosecution; for the defense, two sexologists associated with the Institute for the Advanced Study of Human Sexuality. An invisible army of support staff backed up the three defense teams. Costs for the defense alone ran well into six figures, supplied by Robert Guccione -- who will likely face further litigation in the Deep South.

Despite the absence of any higy-voltage drama in the courtroom, I must confess that the scene moved me. Others I knew who had testified in obscenity trials had told me about badgering magistrates and lawyers. There was none of that in Boston. The proceedings affirmed the majesty of the law.

Dr. Heroian, a California sexologist, preceded me on the stand. Each year, she testified, a couple of thousand souls came to her for therapy or instruction. She felt that the explicit sex in "Caligula," like films used in her work, possessed eductional and therapeutic value. O'Neill's cross-examination focused mainly on the doctor's formal training (in educational psychology) and her miscelaneous publications. My turn followed.

Homans guided me through the recital of credentials. He then asked whether I had any opinion as to whether the film "Caligula" had any serious artistic value. Yes, I thought it had. Where did such value reside? Well, in the first place, the cast included such eminences as Sir John Gielgud and Peter O'Toole, with his five Oscar nominations, and also that remarkable -- and much less familiar -- English Shakespearan actress Helen Mirren, whom I had more than once seen stirring audiences with the Royal Shakespeare Company at Stratford-upon-Avon.

Who, I asked, could not be moved by the suicide of Nerva, as played by Gielgud: the humane man choosing to opt out from Roman inhumanity? In a long and enormously difficult part, Malcolm McDowel caught the complexity of Caligula that has made this bizarre despot cast a spell on the modern sensibility. Camus had aftedr all written a remarkable play about him; I didn't get to say, as I had hoped, that Camus saw Caligula as an insane misfit who wanted the moon. McDowell's emperor, like Camus', felt only the dead were real.

But what about the explicit sex? I suggested that, had those scenes taken place in Muncie, Ind. -- the setting for "Close Encounters" -- they would have introduced a jarringly inappropriate note. But the Emperor Caligula presided over an orgiastic court unfettered by moral constraints. It is but an easy leap from the pleasures of kinky sex (he is incestuously infatuated with his sister Drusilla) to the pleasures of cruelty.

Judge Elam's court raised no objection when I drew a comparison between the world created in the film and the Shakespeare's Roman world in the relatively little-known "Titus Andronicus." The characters in the latter sup full with horrors: A girl is raped by two brutish louts who cut out her tongue and lop off her hands to prevent her from teling the story. Then they mock her with savage hilarity. Blood bubbles between her lips. In revenge her father kills the two louts and serves them baked in a pie to their unsuspecting mother. Caligula at least drew the line at cannibalism, and, unlike Shakespeare's tragedy, it had some historicity. The Roman history in "Titus" is phony; the Emperor Saturninus of the play never existed. But the play, like the movie, showed the beast beneath the skin in the civilization celebrated for law and Cicero and aqueducts. Rome in "Titus" is "a wilderness of tigers"; so is it in "Caligula."

My pulse quickened as I braced myself for cross-examination. O'Neill looked up and paused. No questions. I felt at once puzzled, let down, and (if truth be told) relieved. But the defense was jubilant. Rightly or wrongly, they concluded that the D.A. had decided he couldn't shake my testimony, and so threw in the sponge. On the way out O'Neill and I exchanged friendly glaces, and, if I remember correctly, I shook his hand and mumbled something about his probably not wanted to be late for lunch.

I had had my day in court. Nobody, I reflected, had asked me if I enjoyed "Caligula." At the restaurant in the Parker House hotel, a gang of us, including lawyers and wives, the defense coordinator, and the sexologist, tucked in to Parker House rolls. Spirits lifted; glasses clinked. As I left, Dr. Heroian threw her arms around me, California style, and gave me a kiss. I caught an early plane back to Washington, a little wiser about the workings of justice, and, on the whole, exhilarated.

A week later, in a formal decision which rested on the political value of the film -- its portrayal of how absolute power corrupted in Caligula's decadent realm -- Judge Elam ruled for the defense.