An hour before showtime, James Theadford Weis slips in to case the joint.

"I look everything over," says the personable, dark-haired man who knows more about mascara than a safecracker knows about combination locks."What I'm searching for is the right location to get the job done - a special area where I can spread out my stuff. It has to be out of the way . . . you know, things get crazy backstage when a new collection opens. Once I've found the right spot, I tack up my sketches and then I'm ready for the first one to arrive."

The "first one" he's talking about is the model who shows up the earliest. Weis, one of New York's top makeup artists, specializes in beautifying beautiful mannequins who sashay down runways. He normally sets out the tools of his trade at five big fashion presentations each season in New York, which means there's scarcely time to grab a muffin at the deli between jobs. But, no matter, the man seems to thrive on pressure.

"The more we have to make up the merrier," he says brightly, beginning to outline the perfect lips of Ford model Dianne de Witt.

Weis, who's 41 but exudes the energy and enthusiasm of a teenager, knew what he wanted to do with his life from the time he was 5. That's when hairdressing caught his eye - and from that moment on, cosmetology in one version or another has been his passion.

His parents probably would have preferred a more traditional and down-to-earth career for their son. Something like a doctor or lawyer or banker. But glamour was what he loved. And, he says with a smile, his dad really couldn't say too much. After all, he sold beauty and barber supplies!

So, Weis became a hairdresser. His expertise caught on fast. He made news in Atlanta salons; appeared on television; was featured in magazines.

Eventually, the beautician landed in Los Angeles and became Western creative director for Glemby Salons.

"My job was to motivate other stylists and come up with fresh ideas," Weis says. "It was all right at first, but boring after a while. I started doing more and more print work to find new creative outlets. As I worked with models, I realized they needed as much help with makeup as they did with hair. I started playing around with lipstick and blusher - it was fun! The models who were good at makeup taught me a lot . . . a whole new world opened up, and that's the way I think careers should be. You always have to keep seeking new outlets and new ways to express yourself."

Seven years ago, the makeup artist started his own company, James Theadford Weis Makeup and Hair. He began going to the European fashion capitals and met the man who presently represents him, William J. Knight.

"Billy was a knife salesman when I met him," says Weis. "I could tell he had ability, so I hired him as my secretary. Soon he was booking me into the top shows, and we began expanding the company."

Today Weis and company are among the most highly regarded professionals in the fashion business. Among his Italian accounts have been such biggies as Gianni Versace and the Missonis. New York accounts have included Mary McFadden, Eleanor Brenner, Rebecca Moses, Andrea Jovine, Hino and Malee and Carolyne Roehm. Mugler has hired him in France, and Parisian Countess Jacqueline de Ribes also hired Weis for her American showing several seasons ago.

Frankly, says the makeup expert, the collaboration with de Ribes was not a pleasant one.

"The colors I used turned out to be too strong. On the runway our faces outshone the clothes. The designer was not pleased!"

Much more successful has been his association with Carolyne Roehm, one of the brightest designing stars on Seventh Avenue. There's real rapport, Weis says, when the two meet to discuss an upcoming show. And that's what it takes to make every aspect of a fashion opening successful.

Preparations for the fall opening staged last April in New York began a month in advance. Weis met with Roehm, studied sketches and fabric samples, discussed her target market and began playing around with colors that could best enhance the clothes.

That's the hardest part of the process, he says - mixing just the right makeup shades from the vast artist's palette. Once colors have been blended and a mixture that's current and right seems to have been achieved, he again meets with the designer to get approval.

When the combination of lip and cheek shades in relation to eye shadow tones seems right, it's tried out on a sketch that Weis draws himself. Then it's put on an actual model. Modifications and changes are made along the way.

For the Roehm fall collection, blue-black tones once predominated in the makeup palette. By the time models came down the runway, Weis had changed to blue-orchid.

"We called it `Black Orchid,' though," says the makeup artist. "That sounded more exotic. We usually do name the makeup that's done for a show and we have a lot of fun with it. One spring season we called Carolyne's palette, `Pink Lemonade.' "

For the Roehm fall show, Weis and his assistants didn't do the coiffures. But they were incredibly busy anyway, making up 20 models. The usual procedure is to allow the mannequins to put on their own foundation. Then Weis and company concentrate on eyes, cheeks and lips. Special attention always is given to the under-eye area. Dark circles are death on the runway, emphasizes the makeup artist, and he has developed some tricks and potions to hide even the most stubborn lines.

A few mannequins, though, will have none of it. They insist on doing their own makeup. And he has learned over the years to cater to their wishes.

"I give them sketches of the colors to use and the techniques I'm advocating and hope for the best. Sometimes I'm not too pleased at the result. But a few are real prima donnas - and I've learned not to argue."

Most of the models Weis deals with, however, are true stars - in the best sense of the word. They're cooperative, pleasant, charming, eager to utilize his expertise - and the makeup artist has learned to think of many of them as dear friends.

It's like a family, he says. You see them in Europe. You see them in New York. You see them on print assignments all over the world. And there's a happy reunion every time you meet; sadness at parting.

"When a show's over, I always feel like the little kid who has anticipated Christmas and then has a big letdown the day after."

Weis wins models' hearts by giving them little gifts - special compacts filled with the makeup colors he has concocted to go with a particular collection. They love the personal touch.

As for the makeup artist, you can tell he loves giving presents. And you can tell he loves his job.

"I have this motto," he says, dabbing gray shadow beneath model Dianne de Witt's beautifully arched eyebrows. "Either love what you're doing or find something else. Life's too short to work at something you don't enjoy."