IT'S NOT EVERY day you meet a certified genius, but here's John Sayles--filmmaker, novelist, sports fan and resident of Hoboken, N.J. Last month, the MacArthur Foundation certified him with $30,000 a year, tax free, for five years. There are no strings to a MacArthur Prize; it is given solely to support extraordinary talent.

Now, Sayles says, when anything goes wrong in Hoboken, they say--"Ask John--he's the genius."

It was "The Return of the Secaucus Seven," a poignant look at 1960s people turning 30, made for $60,000, that announced his talent, winning him the Los Angeles Film Critics Award for best screenplay and putting "Secaucus" on Time magazine's Top 10 list for 1980. His current film is "Lianna," also shot in 16mm, an exploration of what happens to a faculty wife who leaves husband and kids for a woman. He also has written two critically successful novels, "Pride of the Bimbos" and "Union Dues." The first short story he ever published, "I-80 Nebraska," won an O. Henry Award.

At 32, John Sayles is cloaked in glory. But because he is 6-foot-4, with an open-faced, short-sleeved air about him, the cloak does not quite reach the floor. He seems to have sneaked in through a side door only to find that the party he has crashed was in his honor all along, and that nobody minds about the alligator in his past, or the werewolf or even the piranhas.

The call of genius came late at night.

"We were mixing 'Baby, It's You' his next film, a romance that opens next month for a test run in Seattle when the phone rang," Sayles said. "I had insomnia, we had been running the picture backwards and forwards all night, and I hadn't applied for any MacArthur grant. The guy on the phone told me how much it was, and it was tax free, and I said, 'Okay, thanks.' He was disappointed that I didn't yell, or anything. The poor guy. I must've sounded like he was telling me my car was ready at Midas Muffler."

Sayles wrote "Lianna," with its lesbian subject matter, five years ago--before he wrote and directed "Secaucus Seven."

"I wrote it way before 'Making Love' or 'Personal Best' came out," Sayles said. (The first was about a husband who leaves his wife for a man; the second, about young women runners in love with each other.) "In fact, after I'd finished 'Lianna,' I met Scott Glenn when they were shooting 'Personal Best' and asked him what it was about. He said 'track and field,' and I believed him."

Sayles makes no claims of insight into homosexuality, but says he wanted to make a movie about a woman facing a difficult choice, and coping with the problems that result. "I got my ideas from a lot of bad custody battles my friends had gone through, and what I'd seen of women coming on the relationship market at age 34. 'Lianna' is about changing, more than anything else. It's about where do we go from here.

"Sure I was expecting people to say, 'What do you know about this?' But when word got out, gay women came and said, 'Here're some ideas you can use, here are some clich'es to watch out for because they're not true, or no longer true.' We got some nice input that way."

The subject matter didn't help the $300,000 fund-raising effort, which took 3 1/2 years. "People kept saying, 'Why don't you make a werewolf movie?' Well, I can write a werewolf movie in a month. But this was one I was going to direct, and that takes a year of your life, and for all the effort, you feel like making something that probably wouldn't get made at all unless you did it."

The alligator in Sayles' past is "Alligator," a genre picture about a large, toothy amphibian lurking hungrily in the sewer system of Los Angeles. It was pretty good, because Sayles has a knack for effortless dialogue, an utterly convincing sense of dramatic pace (even when the pace is a measured slosh through knee-deep improbabilities) and a highly marketable ability to make cardboard characters actually seem to bend at the joints.

He also wrote "The Howling," a gruesomely effective werewolf movie for which he devised a climactic transformation-lovemaking scene that Lon Chaney Jr., wherever he is, must be eternally sorry to have missed. (Boy meets girl werewolf in fire-lit glade; fur flies, fanning flames.) "The Lady in Red," a trap-Dillinger gangster movie, also bears his writing credit. "It's very popular in Europe, where they still believe in gangsters."

Working for Roger Corman, the legendary producer of quickie movies, Sayles became known as a pro who could write an actor-proof horror flick fast, deliver dialogue on demand and get the job done, as he did with "Battle Beyond the Stars." "Piranha," a "Jaws" spoof, was one of those, but somewhere along the line it lost continuity, dramatic tension and interesting characters, and came up pretty short on fish, too.

What happened?

"Actually," Sayles said, "I thought Joe Dante, the director, did pretty well considering the problems. It was too bad they had to darken the screen every time the piranhas come on, so you couldn't see the wires. What happened was that three days before shooting began, they cut Joe's budget by two-thirds. That's right, two-thirds. Also, the lake they were supposed to use was just about dry, because Los Angeles was having the worst drought in four years.

"I know, I know--there weren't many piranhas in the picture. But I did write them. In the screenplay, you're always returning to them, cutting away to the piranhas . . . they're eating things, they're growing stronger, they're menacing people and so on. But when the budget was cut, they just shot as much as they could each day, and the parts of the script they didn't get to, they didn't get to. The same thing happened in 'The Lady in Red.' Sometimes scenes you write just don't materialize on the screen."

Sayles' enthusiasm for movies continues high, however. His two favorite movies of the year were "Gandhi" and "The Road Warrior": the first a celebratory biography, the second a bit of Australian mayhem starring Mel Gibson as a walking compound fracture of post-World War III society. "Not a very likely double bill, is it?" Sayles said apologetically.

He also likes to act. As a student at Williams College and in summer stock, he told American Film Magazine, he was apt to play "large, retarded people." But lately he has had more fun. He was one of the gang in "Secaucus," had a small part in "Piranha" and got to play a morgue attendant in "The Howling." In "Lianna," he is a jockish teacher who attempts to bed Lianna after she leaves her husband. At first horrified by her immunity to his well-practiced charm, he feels much better when he discovers the only possible explanation: She's a lesbian.

Because he "hated the thing," he has now shaved off the large and unruly mustache affected for that lover-boy part. The Sayles sideburns, long and indefinite of termination, are still there. But are they the same length? He is tall, charming. Why does he look, then, as if he has been sleeping under his truck?

Sayles is currently trying to raise $1.2 million to make a movie called "Matewan," a story of mineworkers vs. mine owners in 1920 West Virginia. "Matewan ends in a massacre," Sayles said, "but so few movies deal with violence well. It's always like 'Bambi,' where the deer have big eyes, but the hunters' dogs have little x's for eyes. What's interesting to me is that in any war, the soldiers who get killed on both sides are equally human."

Now it's off to Miami, for more research for his next novel, "Los Gusanos" ("The Worms"), which is about Cuban exiles. "They've never really been allowed to play with a full deck, you know."

After "Lianna," comes "Baby, It's You." For this film Sayles had $3 million to spend, and the setting was Trenton in the mid-1960s. A college-bound Jewish girl (Rosanna Arquette) and an Italian boy who doesn't finish high school (Vincent Spano) struggle with growing up and destinies apart. Whether "Baby, It's You" gets a wide distribution by Paramount depends on how the people of Seattle and other selected cities react to it.

Sometimes, of course, genius is not appreciated in its own time. Oh, that genius stuff again, Sayles' eyebrows seem to say. He's from Schenectady, N.Y., the son of teachers; he lived for a while in East Boston, and he settled in Hoboken. Hoboken is just across the Hudson from Manhattan, but the river is very wide. What Hoboken lacks in glamor, however, it makes up in basketball courts.

"You know, the MacArthur is a funny award. You can't apply for it. They check you out for about a year. My friends now tell me people were asking about me, but they weren't supposed to say anything. The MacArthur people don't want you to spend it all at once, either. They send you a check every month, so you'll be sure to pay the rent. And it does pay the rent, mine and several other peoples'.

"But the thing is, I could pay the rent already. That's why I wrote all those screenplays."