Peter Ustinov is not an actor; he is a cast of characters.

Paramount Pictures showed him off the other day at the Sherry-Netherland to puff "Death on the Nile," running the press past him as though he were the Tutankhamon exhibit. A squadron of Burt Reynoldses with vests and Munchkin lapels shuttled him back and forth between two rooms like a locomotive piston, fueled with a succession of Perrier waters. Each interviewer got 30 minutes, minus traveling time.

Exhausted and ready for lunch. Ustinov kept slipping into lightning Italian. He was talking about his troubles with Italian Customs, and naturally he had to play the Customs man as well as himself. A minute later he was doing I. S. Johar, an Indian actor whom Variety flipped over. Then he drifted into a story about his arrival in Egypt for the film.

"At the hotel this man kept staring at me with a look of profound shock and horror (Ustinov's plump, intelligent, bearded face registered profound shock and horror), and I asked him what was the matter. He said he had seen me on TV the night before and here I was in Egypt, I said, 'Well I can hardly help it if you saw me in Liege last night.' At that he paled (Ustinov paled) and said, 'As it happens, I am from Liege. I'm the public prosecutor there.' And he turned his back on me and never said another word."

The actor, famous for his epicene villains and voluptuaries, from Nero to the vacillating Captain Vere in "Billy Budd," which he also produced and directed, especially enjoyed doing Hercule Poirot in "Death on the Nile."

"That sort of character is dangerous on film," he said. "Agatha Christie wasn't very specific about Poirot, you know, she mostly describes what he wasn't. He works better on a camera picking up every detail."

Nevertheless, his Poirot has an authority that Albert Finney's lacked in "Murder on the Orient Express." Where Finney was merely dandified, Ustinov creates an untidy man struggling to keep up a dandified appearance. He plays the great detective like a somnolent tomcat.

"Poirot was an interesting problem. He's a rigorous, spare kind of man. He says nothing, he sits there, solid and massive, waiting. Like Maigret without the pipe. (Instantly he is Maigret to the life, puffing an invisible pipe - with a curved stem.) He is a conceited man, something of a bore perhaps, but when he's working, there is a complete stillness about him."

Once when directing a film, Ustinov had a whole roomful of characters sit perfectly motionless so long that the audience thought it must be a freeze-frame. When someone finally did move, it was a shock.

As a writer - 18 plays, eight screen-plays, four books - he sometimes is allowed to exbellish a script to enrich the character he plays. In "Topkapi," he took Eric Ambler's delightful jewel-thieving anglo-Egyptain scamp and added a certain heroic dimension that won him an Oscar in 1964 for supporting actor.

By the way, he won another Oscar in 1961 for "Spartacus" and has two Emmys and other awards including a CBE from his native Britain. He was born in London 57 years ago but was, as he points out in his autobiography, "Dear Me," conceived in Russia by his Russian mother and German father.

Novelist, playwright, producer, director, cartoonist, actor in 30 films and 14 plays, he prizes most of all his UNICEF award for distinguished service this year.

"I arrived here via India on a UN tour upcountry Thailand, Australia, where I did a one-man show for nine days, New Zealand and Kenya. I was petrified by that audience, 3,000 people (he has lapsed into an Aussie twang by now), and a night in a nightclub in Auckland. It was my second nightclub stand: the other was in Quaglino's in London. I hated it."

Standup performances are something like the bill ring, he observed. "An audience is rather like a bull - not the brightest creature around, but it can get violent."

For UNICEF he does TV spots, galas, judges art competitions and checks on the progress of programs in the field. He has been working with the UN agency for over a decade. "My interest," be writes in his autobiography, "grew out of a dissatisfaction with a life of the merely amusing, of the merely diverting . . . It may well be that my consciousness of children's needs was fixed by my own observation of a growing family . . . I became conscious of the need to give affection without an immediate ability to do so."

He has four children, has been married three times.

However dissatisfied he may be with the merely amusing, the fact remains that Peter Ustinov makes his living by being funny. And funny he is. Remember his German U-boat captain on the Steve Allen show, with his cap on backwards and his fingers making crosshairs in front of him? Remember him in "Romanoff and Juliet" (which he wrote), solving the Cold War his "balance of feebleness?"

He is even funnier before a British audience, which tends to have more in common than a random American audience and thus is more susceptible to local references, word play and wit in general.

"We censor humor in the West," he said, politely not specifying America.

"I think we censor ourselves. You find yourself taken out of context in some news report, so you become chary. And bland. The difference between Russia and America is that in Russia they put everybody into the same tank but in America they put everybody into the same tank and as they go in they are all informed (American Midwest accent) that they got Rights."

People always ask him if he wishes he had concentrated all his talents in one medium. He always says no, he regards them all as part of the same job.

But it is for the acting that he is best known today. And just possibly, he may be heading for a mini-career as Hercule Poirot. The "Nile" producers plan to film another Poirot story after doing a Miss Marple mystery, and though nobody is admitting anything. Ustinov could well be asked for an encore.

"I wouldn't mind doing another," he said, "but I don't want to turn into a Charlie Chan."

The half-hour was up. Suddenly Ustinov was talking about being caught jaywalking in India. Indian traffic cops, he said, treat you as if you were a 15-man street gang.

And now he was the Indian traffic cop, making a fury mask and waving his arms and shouting. "Oh say! Hi' You there! . . ."