bout 10 minutes into the somewhat eccentric performance of "Pirates of Penzance," which is playing through the summer aboard the four-masted barque Peking at the South Street Seaport, the shrill nursemaid Ruth, with red hair and frumpy nightgown, darts on stage and begins tormenting the lead character, Frederic, who is played by Colin Hanlon.

A few moments later, the Patton- like Major General Stanley gallantly arrives to search for his lost daughters.

Both the nursemaid and the general are played by Martin Van Treuren, a 25-year veteran of the regional, Broadway and Off Broadway theater, who has played mostly supporting roles in musicals from "Camelot" to "Jekyll & Hyde."

The loose adaptation of this Gilbert & Sullivan chestnut has won positive reviews, and Mr. Van Treuren in particular seems to embody the zaniness of a production that makes references to Viagra and "Aida" on Broadway. Lawrence Van Gelder in The New York Times gave Mr. Van Treuren credit for "most of the evening's performing fun."

Mr. Van Treuren, a fit 48, has worked alongside Richard Harris in "Camelot" at Wolf Trap, in Virginia, and he has been directed by Tommy Tune in the first national tour of "Grand Hotel." On Broadway he has played two supporting roles in "Jekyll & Hyde" and was in the cast of "Me and My Girl."

In "Pirates," his general is a stiff and tightly wound martinet while Ruth is a delicate, though shrill 6-foot-3-inch magnolia. Mr. Van Treuren acted like neither while he worked his way through a sandwich at the Edison Hotel Cafe, commonly known as the Polish Tea Room, one afternoon before a performance.

This unorthodox Gilbert & Sullivan has allowed plenty of room for improvisation, he explained. And for him, that is a major attraction. "We deal with weather and the seaport," he explained, with passing ships and their indelicate, inopportunely timed horns regularly threatening to upstage the performers.

"I love putting your little mark on it," he said.

The liberties taken with the original script are so prevalent, in fact, that the show includes a scene in which Gilbert & Sullivan rise up from the dead to complain.

Mr. Van Treuren started going to the theater during high school in Hawthorne, N.J., and there he found his road in life. In particular, the now-legendary 1971 Broadway production of "Follies" influenced him.

"It was the beginning of the high- concept musical," he said. "At the time, I wasn't sure what I was going to do. Then I saw it and said, `I have to do this.' The show took over my imagination."

To play Ruth, he said he adopted Maggie Smith's voice because he has always appreciated its tartness, something he thought would work perfectly in the character of an elderly English nursemaid.

Some people in the audience do not realize he plays both parts. He credits Dawn Robyn Petrlik, who designed the show's props and costumes, for that. He said her influence transcended her job description. "She really helped me create the character," he said.

Mr. Van Treuren speaks readily about the teachers in high school and at Montclair State College (now Montclair State University), in New Jersey, who helped direct his interest and talent. He performs something of the same role with younger actors, like Mr. Hanlon, boosting their confidence when need be. "I am fairly new to this whole scene," said Mr. Hanlon, 23, who is in his Off Broadway debut. "I am very comfortable asking him about things in the business and about acting. When I ask him advice, he lets me try to figure it out myself."

Mr. Van Treuren said: "It's thrilling and brilliant when you can connect with an audience. You have these moments where you know you've clicked with them." And he has been around long enough to see a special role, or set of roles, when it comes his way.