Edith Schy cased the check-out desk with a practiced eye, watching patrons return video cassettes they had borrowed from the Fairfax County library system. Within minutes, she had her prize: a cassette of "Body Heat."

"If you wait long enough, you can get some good things," said her neighbor, Lisa Deason, who had driven with Schy to the Library Media Center in Springfield to pick up their weekend supply of free entertainment. "I think it's a wonderful service."

In Fairfax, as in scores of other localities around the region and the nation, people are finding they can borrow the movie "Halloween" as easily as the collected works of Shakespeare--all for the modest cost of a library card. That compares with commercial outlets that charge membership and cassette rental fees. The recent popularity of video cassette recorders, say national library officials, is prompting a miniboom of cassette buying among librarians eager to keep up with the technological trends.

That's been welcome news for video-hungry consumers, who have lined up in droves to borrow their favorite first-run and classic movies--such popular favorites as "Bedtime for Bonzo," "Barbarella" and "Star Wars."

Still, not everyone is enthusiastic about the video revolution hitting the public library. Fairfax Supervisor Martha V. Pennino (D-Centreville) has asked for an investigation of library video policy, saying it is not proper for the county to spend tax revenues on R-rated cassettes. She also complains that library rules allow children to borrow R-rated cassettes without parental permission.

"There's a great deal of difference between sitting down and reading a book about a love scene, and seeing it larger than life on the screen," said Pennino. "I just don't think we should giving youngsters access to this kind of material."

Fairfax library officials are studying Pennino's complaints but presently have no plans to restrict access to the county's collection of about 1,200 cassettes, which is valued at $60,000 to $70,000. They said virtually all their cassette borrowers are adults, many accompanied by children. The county's current policy, which mirrors anticensorship guidelines set by the American Library Association, allows patrons equal access to all library materials, books or magazines or movies.

"The library board and the system take the position that it's the parent's responsibility to decide what a child may see," said county library Director Edwin S. Clay III. "I'm sure this will sound very naive, but what is the role of the library? It's to distribute information--not to limit distribution of information."

Complaints such as Pennino's are just beginning to crop up around the country, national library officials say, and a library system in the Seattle area last year began requiring children to have written parental permission when they watch or check out R-rated movies, after a mother complained because the horror movie "Halloween" was available to children.

Locally, however, library officials said they have heard few gripes about R-rated cassettes. Montgomery County places no restrictions on borrowing, while Prince George's County requires that borrowers must be at least 18 years old. Arlington County libraries offer no R-rated fare, and while the District offers documentaries and children's films on cassettes, they can be viewed only at the library. Alexandria's cassette collection includes only five R-rated titles, and borrowers must be at least 17 years old to take them out.

More frequent complaints have come from borrowers weary of waiting up to two months for their favorite feature flicks. Just this week, Fairfax discontinued its policy of accepting reservations because of the lengthy backups, although the county will continue to honor reservations made earlier. Prince George's and Montgomery allow patrons to reserve cassettes in advance.

"I think it's about the hottest thing to hit the library in many years," said Kent Moore, Prince George's audio visual coordinator. "Let's face it--it's a great deal."