NEW YORK (AP) _ There will be no complaints that character isn't explored thoroughly enough in Richard Nelson's play ''Life Sentences.''

The two-character play, which opened Wednesday night at off-Broadway's Second Stage, is almost entirely a study of Burke, an English professor in an upstate New York college. Edward Herrmann, who can play austere and aristocratic, proves what a fine actor he is by bringing to life a nerd with facets.

Burke talks for the entire first act. Michelle Joyner, better known for film and TV than theater, plays Mia, his 25-year-old live-in girlfriend. The actress, who isn't as skillful as Herrmann, talks through the shorter second act before the two performers have a final, brief scene together.

Herrmann makes one feel sympathy but not disdain for Burke, who is slow on the uptake and very earnest. He overestimates himself in some ways, underestimates himself in others.

His friend Reid teaches lecture classes. Burke prefers small classes ''where the real teaching happens.'' He limited one to 15 students; five enrolled. Mia moved in with him, she said, because the area's schools are good and she has a little daughter. He adds that she probably was kidding.

Burke's emotions aren't shallow. He misunderstood his caustic ex-wife but loves his teen-age daughter - now living across the continent in Sacramento, Mia, her daughter, good books and teaching. He'll be a friend to Reid ''no matter what he does.''

Herrmann's posture is of a lump who can be taken advantage of. It's the actor's triumph that he makes the less than riveting Burke real and interesting.

Mia is an adaptable survivor who, uninterested in education, hangs around the academic world. Joyner plays her a bit too young, often using the inflections of mall-bound teen-agers. Joyner doesn't dig deep but maybe Mia is mainly instinct.

The sole plot development is the offstage arrival of a Polish guest lecturer, a classroom disaster but catnip to women.

The last scene has the couple, side by side in lawn chairs, speak what they think the other is thinking. They're both entirely wrong but still mutually dependent.

''Life Sentences'' began as the one-person ''End of a Sentence'' two years ago on public television. Nelson's ''Two Shakespearean Actors'' and ''Some Americans Abroad'' ran recently on Broadway.

John Caird, who has worked often at the Royal Shakespeare Company, directed. The Second Stage has a small playing area and designer Thomas Lynch used only its center, a square just big enough for a desk, kitchen counter, etc., to roll on, setting the scene.