The minor rumble of a delivery truck outside his trailer on the Paramount lot is more than enough to set Robin Williams off. Dropping the conversation like a napkin off a lap, he jumps to his feet and shouts in a cracker twang, "Lola! Lola! Get the baby! It's a quake!"

Then he looks around, remembers who he is, sits down again and asks the futile question, "Now where were we?" It's a bit disorienting to come upon a fellow with the inventiveness of an Albert Einstein and the attention span of a Daffy Duck, but these are among the qualities that have made Robin Williams at 26 the hottest comedian in America.

Network television would seem hardly the medium for such a manic new talent, but "Mork and Mindy," a situation comedy built entirely around Williams' charms and inspirations, has become the highest-rated new series of the year, ranking fourth - near "60 Minutes" and "Charlie's Angels' - among the top 10 in the season so far.

"It is strange to think if this show is doing well then 25, 30 million people are watching," Williams says. "Whoa! Ohhh. I think about being in a hall with 8,000 people, or a football stadium with 100,000. But 30 million people? That's like 80 football fields. It's frightening." He looks down, almost sorrowfully. "Very Strange."

A momentary calm - but these spells do not last long. Unlike Steve Martin, whose fame he may eventually eclipse, Williams is a ravenously comic off the screen as on; during a single lunch-hour break between rehearsals he bounces through at least a dozen different vocies, from Peter Lorre's to the squeak of a Hollywood tot to Quasimodo to a swish teacher at an acting school for fish ("I worked with Flipper when he was nothing ") to that of his own tight-lipped parrot, who lives at home with Williams' new wife Valerie and an iguana that sleeps under the refrigerator.

The parrot says only "hello," a two-word obscenity ending in "off" and "birds don't talk."

For Williams, the world is an attic filled with an infinite number of unopened old trunks - or a gorilla's cage into which passerby toss an endless stream of marvelous objects. His is jubilant humor with an impulse to anarchy that suggests all three Marx brothers rolled into one.

So he doesn't just play Mork - a prankish sprout who strolled to earth from the plant Ork - but contributes his own lines, bits and new Orkish quirks to each show. "From a quarter to a third" is improvised or invented by him, he estimates, and this helps keep him from going stir crazy within the confines of a weekly series designed by committee to appeal to everybody.

Like Williams, Mork is a chipper, naive misfit with a kid's-eye view of the world. Conversations are held with potato chips, and baloney sandwiches are fed to hungry plants; a broken egg is considered t have died and is buried at sea. Mork must be recharged on his birthday, has a tendency to talk like a supersonic computer and often enters chairs face first.

The native greeting he has brought from Ork is a twist of the ears and a hearty "Nonno, Nonno," a phrase Williams invented and made into a household refrain in one night. "It's the Orkian 'shalom'," he explains.

His degree of control over the program is unprecedented for such a relative newcomer. That's because there could be no show without him. "What they gave me was more or less carte blanche to do more or less what I want within the limits of a situation comedy - bordering on complete freedom," Williams says.

"I don't have script approval as such, but I can change things if they don't work. It's not like he's saying, 'This is s---, write it over.' We just play. I go and hang out with the writers and write with them." Williams gets additional creative kicks by continuing to solo in clubs like the Improvization and the Comedy Store in L.A., although his work with a satirical troupe called Off the Wall may have to end because word of his participation got out and now his fans storm the place.

The kind of thunderbolt fame bestowed by national television does peculiar things to idols and idolators. "The only hassle I had was one time I was playing a softball game for muscular dystrophy. It was a benefit, and about 400 little kids there all came up and mobbed me for autographs at the end. That was a little scary, because they were little kids and you couldn't be nasty, push or shove or anything. But they were pushing each other!There was a little kid in the front going (baby voice), 'Come on, sign it! Don't be an ass!' Incredible!

"One woman - I guess I didn't sign her kid's autograph or I couldn't get to him or something - and the woman said, 'He's not so hot. Some day he'll be doing commercials again.' Whoa! I said, 'I'll sign it, honest, I'll kiss the damn kid!'"

Then there was the guy who came up to him on a New York street during a visit to Manhattan a week ago. "Oh, he was the best," Williams said. "He said, 'Are you him, the guy that's on TV?' I said, 'No, I'm his brother. A lot of people mix us up.' He said, 'No, really, are you him?' And I said, 'No,' and as I walked away he yelled, 'Well, if you are him, get the f--- off TV!'"

It is already Christmas on Stage 28 at Paramount. The prop department has sent over a regulation requisition Christmas tree and the cast of "Mork and Mindy" is rehearsing its holiday show, to be filmed two days later.

"Where's Mork?" one of the actors asks, and Williams, in his perpetual suspenders and a Keith jarrett T-shirt, bounds on to the set with a mock benedictary, "Hare Krishna everybody." While the director talks with the others, Williams suddenly leaps onto a table and turns into Zorro: "Ah, Sgt. Garcia!" he shouts, thrusting an imaginary sword at Mindy, co-star Pam Dawber. "Oh! I've skewed Consuelo!"

Soon he is serenading a tree with "chestnuts roasting on a microwave." He does pushups at the top of the stairs as he waits for his cue and later sums up the script he hold in his hand with, "Mork the pinhead does it again!"

To friends in the studio he will shout out in genuine or faked Italian, Spanish or French - or to a guy standing near the door, elaborate and surely meaningless sign language. "Let's do leaps," he says to Dawber, lifting her with an "Up - ho!" Then he spies a rope dangling from the rafters, pretends to hang himself with it, and announces, "The series is called 'Mindy,' now."

To no one's surprise, the director completely agreed.

Williams has power over the program that old-timers would envy, but he seems to exercise it gently, not with fits or tantrums. He concedes that the time will soon come when his contract will be renegotiated - upward, inevitably - but says his manager takes care of all that so that he never has to play the prima donna himself.

"I have to improvise, just for the sake of my own sanity," he says later. "It's like a little kid saying (baby voice again) 'Lemme do it' (resumes adult voice). Sometimes I'll improvise whole speeches, whole long sections or interplays between people. We saved two endings on scenes a week ago just by letting me go for about a minute extra."

Being around when Williams gets rolling on one of these impromptu binges - especially during a club appearance like the one recently taped for Home Box Office cable TV - is a little like being inside a nuclear popcorn popper as the kernels start to explode. Clearly, "Mork and Mindy" barely exercises his comedic muscles and he knows, if not admits it. "It's getting better though, the focus is shifting," he says optimistically, and he thinks he could stay with the show three, four, of five years - but only "if it goes through transformations, if it grows."

To some, a hit TV series is the answer to all prayers, to others it is a certification of selling out. Williams straddles these extremes. Playing an Orkan may not be what he dreamed of while studying drama on a scholarship at the Juilliard School under John Houseman, but he thinks that if he had stayed in New York after graduating, then "just about now I'd be getting regular work, serious acting jobs. Hopefully. Or, just about now, I'd be taking my life."

Williams spent the very first year of his life in Scotland, then moved with his parents to Detroit and later suburban Chicago. Was he gregarious as a child? "No, only with myself." He becomes Peter Lorre: "I was very outgoing with myself. They don't understand me."

He was once, briefly, a political science major with the wacky ambition of joining the diplomatic corps - "the fantasy lasted one semester." His father always urged him to do what he wanted, Williams says, "but he told me to have another profession like welding to fall back on. My mother - a crazy, wonderful Southern belle.A mad woman. A Christian Scientist and a heavy party lady. It's a weird combination. She doesn't drink but she loves to party.

"A lot of my early act used some of her jokes. Like, 'I love you in blue, I love you in red, but most of all, I love you in blue.' That was one of her poems."

He almost ended up "scooping ice cream in an organic ice cream parlor," but comedy called. It screamed. Comedians may differ widely in styles so that an original upstart like Williams wouldn't appear to have much in common with any other - he hates being called "the new Steve Martin," for instance, because "I don't think I'm anything like him." And yet there are common bonds among all clowns: irreverence; bravery in facing audiences: masochism in facing audiences; and the comic's curse of insecurity.

So when Williams hears that a trade paper gave a negative review to his HBO tape, and no one has shown him the review yet, he gets obsessively curious - finally shouting in a way that suggests a serious strain beneath the jest, "Find that man! Find out who he is! We'll make him eat the damn words!"

He keeps returning to the spectre of this singular rejection. "What exactly did he say was wrong?" Comedians want everybody to want them.

"It's always been a drive to be liked. Lenny Bruce talked about it, it's nothing new: the like-me-love-me syndrome. You want people to enjoy it. Something inside drives you. Comedians have this imbalance that has to be fulfilled, and that's why they're constantly trying to add the other half, that need to be funny. I guess that's what makes some of them very desperate. I don't think I'm desperate in the sense that I'd go to any lengths to get a laugh. Sometimes if I'm dying in a club I'll just end by saying, 'Thank you. If I haven't been funny, relax. The energy's all right. I'll be back. I'm a Zen comedian. It doesn't matter.'"

He looks around the room and spies a bottle of Perrier. A hillbilly demeanor overcomes him: "Want some more of that frog water? I'd like to see a commercial on television." He shifts into a Shakespearean respectable voice: "You know, whenever I want to have an enchanting evening, I have this frog water, Perrier. For the tragically hip."

What he wants to do: a movie (he is not writing a script), concerts, more club dates and an album in the works. What he won't do: play Las Vegas or appear on "The Hollywood Squares."

The choices are all his and he has enough energy to run a ferris wheel for the next 100 years. Whether TV will burn him out quickly or provide him with a base on which to build remains to be seen. For now he is the freshest, most inspired and industrious human leprechaun since the early days of his hero, Jonathan Winters.

A tape recorder sitting next to him has been a silent obedient listener. Now he turns to it as if it were a kindred spirit. "You understand, though" he tells it. "No, what do you know? You're made by Japanese people! They understand you."

He looks up and appears fleetingly sheepish. "That's a lot of tape I've wasted. You'll take it back and say, 'That's not funny! It seemed so funny then.'"

He picks up a lumpy carryall with his props, harmonica, book on meditation and Zap comic books inside and ambles back to Stage 28. They are waiting for him near the lighted tree the way kids wait for Snata on Christmas Eve; he is their child and their jester and their captured alien, and his bag of tricks seems bottomless for now.