AT FIRST GLANCE at that wonderfully extended first glance over Nicollette Sheridan (the very titular Sure Thing). Rob Reiner's new movie looks like some hybrid of a late night blue movie on cable, an updated It Happened One Night with a junior prom live! soundtrack. But the putative story line--incompatible boy meets non-patible girl, girl rejects boy, girl and boy get thrown together on long trip, fall in love, break up, stumble back in love, kiss in moonlight, roll credits, sell cable rights, ad clicheum--stays mercifully irrelevant in what soon becomes a veritable impros comedy feast coated in Rob Reiner's relentless but unjaded satire.

We all knew Gibs in high school, or imagined we did. Gib, of course wants it--going as far as traveling from the Ivy League to California to get it--but Gib is as much the aspiring philosopher as the horny teenager, a kind of Voltaire with a two hour-long hormone overdose. Let other high school chums compare the relative merits of different Ray-Ban tints or Coppertone 6 versus Coppertone 4, but Gib still savors the meaning in life "Who invented liquid soap and why?" No talk of etching or astrological signs in his romantic approaches; witness his favorite pickup line: "Imagine a sexual experience so intense it could potentially change your political views."

What makes the Gibs of the world so charming and The Sure Thing so successful is not so much their brilliant insight (not to deride the value of essays on "How to eat pizza without burning the roof of your mouth") but their natural energy and spontaneity.

Just what then can our man Gib see in Alison Bradbury (Daphne Zuniga), whose appointment book schedules all but the fluttering of her long lashes, who sees college as an ivy-adorned clearinghouse to law school. Who really cares if she really is a two-time delegate to the model U.N. in New York City?

This aspiring Perine Mason and the lustful philosopher get thrown together by the two great college mixers, the introductory writing class and the vacation road trip. In first year writing class. Alison discovers that her writing is as dry as Gib's sex life, while Gib realizes that, Faulkner excepted, writing requires something resembling sentences Their teacher. Professor Traub (Viveca Lindfors), runs her writing class like a laboratory in life, passing by the usual that which distinction for such advice as "Talk to people whose clothes are not color coordinated Make love in a hammock."

Rebuffed by Alison and bummed by Ivy life in general, Gib, tantalized by his best friend lance's tales of a sexual sure thing ("Every relationship begins with a one-night stand") decides to head for the robust UCIA beauty. Since Alison is on her way to swap law articles with her similarly situated boy friend, the pair end up hitching their way to California together, a la It Happened One Night. Somewhere between playing a madman psychotic to scare a lecherous truck driver off of Alison and trying to identify the three categories of junk food. John Cusack manages to make the film into a showcase for his improvisational comedy talent.

FOR ALL THE SEXINESS of its opening and its Calvin jeans ad campaign. The Sure Thing is not a very sexy film. Even in the dream seduction sequences between Gib and the his bikini beach babe, the girl just asks for "More" and Cusack dreamily says "later, later, later" It doesn't sound like love; it doesn't look like sex (of an R-rating quality); and it comes across, in the end, like something of a long, hard to film masturbatory sequence. Good enough. Eroticism is as distant from Cusack's physical repertoire as genuine Voltaire-level perception is from his musings on pizza scarfing. No doubt Cusack could get laughs reading the Peoria phone book out loud. But Donitos Juan need not worry. Were he actually to wear floppy feet and a big red nose while making love, the overall impression would not noticeably change.

Rather than alternating between sexual allusions and sexual illusions in the style of the generic teen flick. Reiner uses the cross country trip scenario as a chance to wink, gently but snidely, at the void that is Middle American culture In smaller but still delightful doses. Reiner offers the same subtle satire that made This is Spinal Tap! the best comedy of last year.

Think of Middle America Didn't you always suspect some buy depot types who asked for a dollar really could offer you change for your twenty? Who hasn't met the nice suburban couple with their laughing gas-like smiles who sing showtunes as they drive down the highway in their sky-blue station wagons? It sounds like stereotyping, as might befit a director first made famous as Archie Bunker's "Meathead" son-in-law, but this flick is simply too much fun to criticize the Styrofoam characters with any relish. Those who appear in the credits with titles like "Girl in Photo." "Frat Guy," "Pick-up Driver," or "Bus Station Bum" are not characters: they are nothing more than props for Reiner's comedic mind, no realer than a red nose or floppy feet.

Nor should they be. The nameless, stereotypical comedy props are Falstaffs for the Eighties, the stuff of a rich comedic vein that runs under and holds up the presumptive romance in this comedy romance Even against Cusack's domination of this running improv show, his co stars pull off a remarkable success time after time findfors in her few on-screen minutes, plays Professor Traub as more than just a wacky writing teacher: she is a kind of Paper chaselaw professor, seemingly about to utter. "Take this dime and call your mother Tell her you will never be a writer."

Billed as a romance, advertised like a celluloid Penthouse forum. featuring a comedy team cast too good for either. The Sure Thing becomes more than any of this it is the freshman movie of that spring.