In some respects a weak vehicle can reveal more about an actor's reserves of talent and personality than a strong one, simply by obliging him to rely on those reserves in self-defense. The new film "First Love" seems such a negligible account of a short-lived college romance that it's impossible to tell what attracted anyone to the material - or kept someone from elaborating the premise more humorously or forcefully.

The original source, a Harold Brodkey story titled "Sentimental Education," might clear up the confusion. I haven't been able to turn it up, but other Brodkey stories on the subject of erotic rites of passage leave me doubting that his meticulously analytical approach could prove anything but agonizing on the screen. What sustains interest in the movie is not the story or characters but the phenomenally attractive romantic image projected by William Katt, the young actor who made a sensational debut last year as the gallant high school dream-boat who escorted Sissy Spacek to the prom in "Carrie."

Elgin Smith, the college boy protagonist of "First Love," cherishes such an unreasonably idealistic concept of romantic love that he sabotages his romance with a pretty coed named Caroline (Susan Dey), who is in the last stages of a prolonged affair with an older, married man and unable to give Elgin all the long-term commitments he craves.

Katt's youthful good looks and diffident personal charm help to soften the esential foolishness and priggishness of the character, which doesn't do much for Katt in return, except serve as a warning against further schoolboy roles. There's no reason why the conflict between Elgin and Caroline shouldn't be interesting, if only it were dramatized with a little attention. Two elements in particular need clarification: Caroline's feelings about the two men she's involved with and the self-defeating nature of Elgin's infatuation, which inspires him to be his own overpossessive worst enemy.

Nevertheless, there is Katt, and the fact that he confirms his appeal in such static, uninspired circumstances must mean something. Heaven knows Joan Darling, rumored to be very despondent about the way "First Love" turned out, doesn't support him with the sort of dynamic pictorial glamour and romantic inventiveness that Brian De Palma brought to "Carrie."

There are token attempts at pictorial lyricism in "First Love," with Katt kicking around a soccer ball in photogenic solitude, the backlighting caressing his blond curls. Elgin's solitude seems one of the funnier affectations in the movie, like Candice Bergen's dateless weekends in the doleful "T. R. Baskin."

Katt is too likable and too luminously handsome to let himself get caught impersonating shy, virginal guys who may not know the score yet. The camera is irresistibly drawn to him. He doesn't need to do anything large to attract attention. Slight flickers of expression, particularly in his watchful eyes and quick, warming smile can convey a powerful seductive charge, and he't got a pleasantly modulated voic!e to augment the low-key sex appeal of his image.

It appears that Katt knows how to make people feel at ease with his exceptional photogenic advantages, and his diffidence is attractive in the same graceful way that Cary Grant's was. Kat is also a seductive rather than aggressive image of masculinity, potentially capable of sustaining and updating the Grant tradition of romantic comedy.

Since Katt may always look at least 10 years younger tha he is, one would hope to see him toughen up and mature in future roles, portray characters who are well out of school and into working and professional worlds, as well as characters who have the sort of guile that Elgin is supposed to be innocent of - the recognition that good looks and charm can certainly be an advantage and might as well be taken advantage of on the most desirable occasions.

No other young film actor has as much star potential as William Katt, and it shines through the images of a picture as hopelessly inert as "First Love."