AS PHARMACIST Joe Glenn climbed a stepladder behind the shelves at Morgan Pharmacy, he muttered, "I sure hope I don't fall because I'm making a movie tomorrow."

"What's that?" the customer said. "I know every picture made in Washington."

So Glenn told her about his bit part in "Diner." It turned out she owned a production company here.

"I tell everybody about my acting," Glenn later confided. "It's one of the ways you find out about work. You wouldn't believe how many theater and film people come in here."

(Morgan's, in the heart of Georgetown, also draws a lot of scuba divers and photographers because that's what the boss, Harold Elwyn, does for fun. But that's another story.)

As a pharmacist who acts, or an actor who supports himself as a pharmacist, Glenn's name is legion. Over the past five years he has played nearly 40 roles in community theaters and dinner theaters as far away as Philadelphia, in industrial and dramatic films and on TV. Considering that dinner theaters pay in dinners and maybe $20 a night and that most non-Equity theaters pay nothing, the competition is terrific.

"For a part in little theater, 25 to 50 people will show up. For every part in an Equity theater production you'll get hundreds. I had seven lines once in an education film and got $200. That's where the money is."

It's not the money, though, that keeps people like Glenn poring over the trade papers and hounding the local casting offices and getting up at 5 a.m. to read a play through one more time and working weekends to leave weekdays free to visit agents. He started in 1950 at Philadelphia College of Pharmacy, on a $5 bet that he couldn't get a part in "Dear Ruth." Later he quit acting to raise a family (two boys and a girl) and be a pharmacist, a life that kept him busy for 20 years. Watching TV acting, he says, lured him back into the theater. Then, not two years ago, his wife died, and he had more time than he wanted.

"I'm getting better parts now. Learned my trade on stage. Reston Theater just asked me to read Willie Loman the central role of "Death of a Salesman" , and something like that sets you up. I'm getting invitations to read parts from places like Arena and Round House."

He landed the Willie Loman part, by the way, but later had to forgo it because of business commitments. It's a role he's wanted to do for years. The length of it never bothered him: Those repeated readings give him the structure of a play, the logic of the lines, and when he knows which character he is talking to, and why, and what happens, the lines themselves are easy to learn.

Nothing is more depressing, he says, than working with someone who has memorized lines like some exercise and gotten off the track. Many's the time Glenn has had to supply the other half of a dialogue. "What, you don't believe I really went to the beach? Well it so happens . . ."

He also sharpens his technique at the Drama Lab and has taken Tony Abeson's Source Theatre seminars and a Folger course. He can do comedy characters, villains, anybody from 40 on up who's rather short and ruddy. Can't sing a note ("my range is M sharp to R flat") though he managed a Scrooge in a musical "Christmas Carol." Wants only to be considered a good, reliable character actor, the kind of actor who can do whatever he's asked to do.

"I visited some Hollywood studios a while ago, and they told me that if I was 6 feet 2, blond and handsome, could sing like Sinatra and dance like Astaire, forget it, there's thousands of 'em around. But they always need a good character actor."

He's done two and three parts at once for struggling dinner theaters (but refused to take tickets and clean tables, too), chipped a tooth in the line of duty, fell off a ladder 30 times in a part. Offstage, he has also dabbled in flying, scuba diving and hang gliding.

"Sometimes at curtain call you're hearing the applause for each character as they come on one by one, and then when you come out it goes up a notch, they're all clapping to beat the band, and oh man, that's what it's all about."