'No Small Affair': a small movie

Take a couple of popular pastimes like photography and rock music. Add a love affair. Throw in a teen-age boy's coming of age. You have makings of a crowd-pleasing movie - if you keep the plot surprising and the energy level high.

Jerry Schatzberg, the director of ''No Small Affair,'' doesn't manage the last two requirements. A stylist who worked with still pictures before moving to cinema, he cares more about capturing the visual charms of San Francisco than smoothly unfolding the happy-sad story. The result is friendly but bland, rather like the main character himself.

He's a would-be photographer who doggedly plies his hobby despite the distractions of school and his goofy family. Roaming the streets in search of good material - he only shoots objects, because people are too unpredictable - he accidentally snaps a young woman whose image starts to haunt his fantasies. He tracks her down and weasels his way into her life, which turns out to be unhappy at the moment, since her singing career is on the rocks. He tries to help, and his crush on her gets deeper by the minute.

Jon Cryer looks exactly right as the hero, whose earnest face would suit ''Revenge of the Nerds'' just fine, and Demi Moore is persuasive as the heroine. The lively supporting cast includes Anne Wedgeworth as the protagonist's flighty mom, Jeffrey Tambor as her boring boyfriend, Elizabeth Daily as big brother's fiancee, and George Wendt - who pops up all over nowadays - as a lecherous nightclub owner.

If these talented people had worthwhile things to do, ''No Small Affair'' would be no small movie. But the action has many weak moments, and the subplots are trite, especially when the trendy bachelor-party scene arrives. Too bad the screenplay, by Charles Bolt and Terence Mulcahy, doesn't live up to the cast or to Vilmos Zsigmond's careful cinematography. 'American Dreamer'

''American Dreamer'' goes for thrills as well as laughs, and pays off mildly well in both departments - but drags on long after its inspiration has run out.

JoBeth Williams (competing with herself in the wretched ''Teachers'') plays a henpecked homemaker with a talent for writing. When that talent wins a contest and a trip to Europe, she expects a week of quiet Old World fun.

But the movie trips her up with one of Hollywood's creakiest conventions - a blow on the head that scrambles her brain. She gets the notion that she's not an ordinary woman after all, but a heroic sleuth right out of a romantic novel. She stumbles on a real-life crime scheme quicker than you could say ''hokey and contrived,'' and spends the next few reels getting ever further out of her depth.

This is fun for a while, especially when director Rick Rosenthal tosses in some deftly edited slapstick. The elegant cinematography is by Giuseppe Rotunno, one of the best. And the cast is strong - Tom Conti as a writer who becomes the heroine's henchman; Giancarlo Giannini as their mysterious and possibly dangerous quarry; and James Staley as a hilariously dull husband back in the States.

But the movie has too many endings, and none of them come soon enough. Someone should have torn a few pages from the screenplay by Jim Kouf and David Greenwalt, or at least speeded them up. Even the suspenseful climaxes seem tired after so much delay. 'Not for Publication'

''Not for Publication'' comes from director Paul Bartel, who thinks humor and outrage go hand in hand. He has been peddling both since his ingenious film ''The Secret Cinema'' more than a dozen years ago. His new opus has the same brash wit and bemused cynicism as the recent ''Eating Raoul,'' which had a fairly visible mainstream run before settling into its niche as a ''cult'' attraction.

The heroine of ''Not for Publication'' works for the mayor of New York by day , a sleazy newspaper by night. Her goal is to wrest control of that absurd tabloid - the Informer - and make it the respectable, crusading Enforcer it used to be when her daddy owned it. In her way stands an editor who believes in the ''three S's'' of journalism: sex, scandal, and sin. On her side is a befuddled photographer who means well even though he's not certain whom he's working for.

Bartel plays all this mostly for laughs, giving the plot a semiserious tone only during the last third, which is the movie's weak spot. He has great fun with names - the editor is Mr. Troppogrosso - and when the story wears thin he throws his heroes into a song-and-dance routine with animal costumes. While the humor is sometimes as coarse as the Informer itself, it's all in fun, and, given the squalid subject matter, the treatment is downright restrained by current standards. The ending is generous to a fault, with virtue rewarded and vice - well, tamed if not exactly vanquished.