AT THE MOMENT most of the interesting writers, directors and performers in the American movie industry seem to be starting or completing a comedy of some kind-farcical, dramatic, sentimental, satirical. I stopped counting as the number of arguably reputable projects neared 100.

A glance at the top 20 attractions of 1978, with its preponderance of comedies-"National Lampoon's Animal House," "Heaven Can Wait," "Hooper," "Foul Play," "Revenge of the Pink Panther," "Up in Smoke," "The Cheap Detective," and "High Anxiety," among them-suggests why.

And it happens that three of the four most successful foreign-language films were also comedies: "Dear Inspector," "Dona Flor and Her Two Husbands" and "Bread and Chocolate." The first successful import of 1979 has been "Get Out Your Handerchiefs," a remarkably original comedy.

"Comedies have a long life," Mel Brooks reminds us. They're often durably popular as well as immediately welcome. Brooks' biggest hits, "Blazing Saddles" and "Young Frankenstein," have been revived lucratively every few years. In the right location, like the Outer Circle in Washington, foreign films as popular as "Cousin, Cousine" and "Bread and Chocolate" can run indefinitely.

Comedies also stand ready to fill the thematic trends gap in Hollywood, and it's a good thing because thematic trends tend to be tenuous and shortlived.

The uneasy theme of the war in Vietnam has been fitfully dramatized in only a handful of movies, and may already be a thing of the past now that major Academy Awards have gone to "The Deer Hunter" and "Coming Home." The only Vietnam movie left on the horizon is Francis Coppola's troubled "Apocalypse Now."

The subject of juvenile delinquency, a going concern in the '50s and always available for updates, has reemerged, perhaps as a result of the rumble sequences in "Saturday Night Fever." Several movies about youth gangs went into production simultaneously, but the violence attending the opeings of "The Warriors" and "Boulevard Dreams" has probably cooled this coincidental trend. Universal recently announced that it was postponing the release of the third picture in the cycle, "Walk Proud," originally titled "Gang!"

The more respectable "women's pictures"-"Julia," "The Turning Point," "An Unmarried Woman," "Norma Rae"-seem to have grown out of a genuine but cautious thematic commitment at 20th Century-Fox. But by and large the so-called return to women's pictures has been a retreat to idle tearjerkers, with the heroines typically immobilized by crippling injuries or neuroses.

The success of "Star Wars," "Close Encounters of the Third Kind" and "Superman" has probably established science fiction and humorous fantasy as the most promising sources of movie spectacle in the future, but they're unlikely to become plentiful, since they require expensive production schedules and budgets.

What is plentiful however, are comedies, some of which also happen to be remakes ("Heaven Can Wait") or sequels (Peter Sellers' return engagements as Inspector Clouseau).

The biggest comedy hit of 1978, "Animal House" also has become the most influential. Its best performers, John Belushi and Tim Matheson, were immediately recruited for leading roles in Steven Spielberg's "1941," the most ambitious comedy project of 1979, an elaborate farce about war scares in Los Angeles the week after Pearl Harbor that appears to be synthesizing elements drawn from "It's a Mad, Mad, Mad, Mad World," "The Russians Are Coming! The Russians Are Coming!" and "Dr. Strangelove" with nostalgic period recreations.

John Landis, the 27-year-old director of "Animal House," has a bit role in "1941," which will be released at Christmas. This summer Landis will direct Belushi and Dan Aykroyd in "The Blues Brothers," the first of their projected "Road" comedies, utilizing their musical alter egos, Joliet Jake and Elwood Blues, an act originated on "Saturday Night Live" and sustained with phenomenal success on a record album.

A few years ago Universal began encouraging comedies even more systematically than Fox encouraged women's pictures. The investment paid off spectacularly with "Animal House," which also brought the National Lampoon into mass audience filmmaking with a belated bang. Universal will distribute "1941," a $24-million coproduction with Columbia.

"The Blues Brothers" inaugurates a three-picture deal Universal arranged with Belushi and Aykroyd. In addition, the studio may have solved the problem of how to squeeze another sequel out of Spielberg's "Jaws" by entrusting a spoof, tentatively called " Jaws 3, People O," to the Lampoon crowd.

Although the project has been postponed for the time being, Landis also was signed by Universal to direct Lily Tomlin in a science-fiction farce called "The Incredible Shrinking Woman." The idea sounds wonderful, but it's evidently being reconsidered in the wake of budget estimates and Tomlin's disappointment at the flop of her romance, "Moment by Moment." In the meantime Landis continues to develop a horror comedy called "An American Werewolf in London" and an adventure comedy-fantasy called "Big Trouble."

Universal also has the next Peter Sellers vehicle, a comic version of "The Prisoner of Zenda" directed by Richard Quine.Scheduled to open May 25, the film will display Sellers in three roles: the ill-fated old king of Ruritania, his playboy son Prince Rudolph and Rudolph's cockney double, a London cabbie named Sid Freqin.

Although Sellers has mercifully retired Inspector Clouseau for the time being, he is supposed to star in comic adaptations of "Chandu the Magician" and "Fu Manchu," playing both hero and villian in the latter. Sellers also has the laed in Hal Ashby's movie version of Jerzy Kosinski's "Being There," a comic fable about the rise to prominence of a blissfully ignorant waif, whose impressions of the world derive exclusively from television.

Blake Edwards, the creator of Clouseau, has finished a new comedy called "10," costarring Dudley Moore as a Hollywood composer preoccupied by fantasies of The Perfect Woman, a great nuisance to his sensible, desirable wife, Julie Andrews. Edwards and Moore plan to reunite on a spy comedy called "The Ferret."

Woody Allen returns to comedy with "Manhattan," which opens May 2. Evidently a chronicle of romantic infatuation and disillusion, the film costars Diane Keaton, Michael Murphy, Meryl Streep and Mariel Hemingway. According to rumors, Allen plays a writer who is dropped by his lesbian wife, takes up with Keaton (the estranged wife of best-friend Murphy) but eventually finds more satisfaction with Hemingway, a precocious schoolgir.

Allen's cowriter, Marshall Brickman, has gone on to direct his first feature, "Simon," a comedy starring Alan Arkin as "a manufactured extrater-restrial." The veteran screenwriter Walter Bernstein is making his directing debut on the third remake of Damon Runyon's "Little Miss Parker," a Universal production costarring Walter Matthau, Julie Andrews, Tony Curtis, Bob Newhart and Sara Stimson, a 6-year-old non-pro from Helotes, Tex. In contrast to "Sorrowful Jones" and "40 Pounds of Trouble," the new remake will return to a '30s setting. Curtis, who played the lead in "40 Pounds of Trouble," will have a supporting role in the Matthau version.

Anne Bancroft is making her debut as a screenwriter and director with a comedy called "Fatso," in which she costars with Dom DeLuise. Before starting this production, DeLuise directed himself, Suzanne Pleshette, Jerry Reed and Ossie Davis in "Hot Stuff," a comedy set in Miami about a police scheme to undermine fences with an undercover operation similar to the "Sting" capers enacted by Washington cops.

Mel Brooks will soon be back in harness with an expansive farce, "A History of the World, Part I." Paul Mazursky begins shooting his new dramatic comedy, "Willie and Phil," the chronicle of a romantic triangle involving Michael Ontkean, Margot Kidder and Ray Sharkey, next month in New York. Martin Brest, the promising young humorist who got Sharkey started in the extraordinary student film "Hot Tomorrows," will make his official feature debut on "Stepping Out," an original comedy with Art Carney and George Burns that also rolls in New York next month.

Universal will launch Steve Martin's feature movie career in "The Jerk," a rags-to-riches farce written by Martin, Carl Reiner and Carl Gottlieb and directed by Reiner. The inexplicable success of "The Last Remake of Beau Geste" has encouraged the studio to humor Marty Feldman again. His project, "In God We Trust," evidently built around a TV evangelist, appears as if it might conflict with Norman Lear's maiden effort for Universal, "Religion," but the more the merrier.

To the studio's credit, the undeserved failure of "I Wanna Hold Your Hand" was not blamed on director Robert Zemeckis and his writing collaborator, Bob Gale. After completing writing chores on "1941," they will embark on a new comedy called "Used Cars," dealing with cutthroat competition between utterly unscrupulous car dealers.

Jonathan Demme, the director of "Citizens Band," was also noticed by Universal, where he's now completing "Melvin and Howard," a potentially delightful account of the alleged meeting between Melvin Dummar, played by Paul Le Mat, and Howard Hughes, played by Jason Robards, and its curious repercussions after Hughes' death.

Barbra Streisand and Ryan O'Neal will be reunited as a romantic comedy team in Howard Jieff's "Main Event," presumably one of the crowd-pleasers of the summer. Streisand plays a businesswoman who tries to engineer a comeback for O'Neal, a retired prizefighter.

Burt Reynolds and Jill Clayburgh get a chance to renew the rapport of "Semi-Tough" in "Starting Over," Alan J. Pakula's movie version of the Dan Wakefield novel, adapted by James L. Brooks, one of the cofounders and mainstays of the old Mary Tyler Moore series.

Reynolds, who became the country's top box-office attraction last year on the strength of "smokey and the Bandit," "Semi-Tough," "The End" and "Hooper," is supposed to continue perfecting his comedy technique in a sequel to "Smokey" directed by Hal Needham, a jewel theft comedy called "Rough Cut" directed by Blake Edwards and a romantic comedy called "Seems Like Old Times" written by Neil Simon.

Between collaborations with Reynolds, Needham has directed Kirk Douglas and Ann-Margret in "The Villain," a chase farce that sounds a lot like "Smokey" transposed to the Old West. Until recently, when Michael Cimino and Walter Hill began apparently straight Westerns, the only Westerns on the horizon were comedies: Needham's "The Villain"; Robert Aldrich's "No Knife," in which Gene Wilder plays a Polish rabbi who survives the Wild West only through the aid of a genial badman portrayed by Harrison Ford; Peter Fonda's "Wanda Nevada," a kind of Western "Little Miss Marker" with Brooke Shields as the little miss, and Lamont Johnson's "Cattle Annie and Little Britches" with Burt Lancaster and John Savage.

Robert Redford and Jane Fonda return as a romantic comedy match in Sydney Pollack's "The Electric Horseman." In this updated Western comedy set in Las Vegas, Redford plays a former rodeo star whose life is disrupted when he becomes the advertising symbol for a breakfast cereal. Fonda, presumably functioning like Jean Arthur and Barbara Stanwyck in old Frank Capra comedies, is the reporter who becomes romantically involved while covering his exploitation.

Everywhere you look, there seems to be a comedy in the wings. Even Al Pacino will evidently debut as a farceur in Norman Jewison's "And Justice for all," a serio-comic account of the trials and tribulations of a harried criminal attorney in Baltimore. After several years of specialization in the horror genre, Brian De Palma returned to comedy. His "Home Movies," a satire on sibling rivalry, features Kirk Douglas, Vincent Gardenia, Nancy Allen, Gerrit Graham and Keith Gordon in a project made independently with a crew largely recruited from Sarah Lawrence, where De Palma began his filmmaking career as a graduate student.

The Muppets make their film debut this summer in a film directed by James Frawley of "The Big Bus." John Ritter rides the success of "Three's Company" into a feature comedy called "Captain Avenger." Robin Williams is supposed to make his debut in the title role of "Popeye," written by Jules Feiffer, costarring Shelley Duvall as Olive Oyl and directed by Robert Altman, who has been on the skids but may have time to right himself with "Health," an all-star satire about a health food convention.

"Saturday Night Live" has led to movies for Chevy Chase, John Belushi, Dan Aykroyd and Albert Brooks, whose "Real Life" will open here soon. Now Bill Murray will turn up in "MeatBalls," a farce about a summer camp directed by Ivan Reitman, a coproducer of "Animal House." Murray may also play Hunter Thompson in a loosely biographical camedy by John Kaye called "Where the Buffalo Roam."

Richard Proyor, going strong in the extraordinary concert film of his comedy act, is supposed to team with Bill Cosby in "The Oddest Couple," star in a Neil Simon original called "The Macho Man" and play an updated Sheridan Whiteside in "Southern Comfort," a remake of "The Man Who Came to Dinner." In addition, one can expect humorous stimulation in upcoming spectacles like the sequels to "Star Wars" and "Superman" and the ongoing James Bond series, represented this summer by "Moonraker" with Roger Moore and evidently destined to see the return of Sean Connery in "Warhead," a Bond adventure announced by rival producers. Perhaps the most decisive indication of a comic swing is the return of Jerry Lewis, who is ending an eight-year layoff by directing and starring in "Hardly Working" down in Florida, where he seems to have found a new backer. Lewis is supposed to jump right from his comeback movie into a geriatric "Animal House" entitled "That's Life," with Ruth Gordon as the head troublemaker at a rowdy old folks' home.

If Lewis is back, can Hope and Skelton be far behind? We may yet see them together for that aborted reprise of the Hope-Crosby comedies that was supposed to be called "Road to the Fountain of Youth." CAPTION: Picture 1, On the laugh track are, John Belushi in "1941,"; Picture 2, Peter Sellers in "The Prisoner of Zenda,"; Picture 3, Ryan O'Neal and Barbara Streisand in "The Main Event,"; Picture 4, Walter Matthau, Bob Newhart and Sara Stimson in "Little Miss Marker."; Picture 5, Woody Allen and Diane Keaton in "Manhattan."