For 18 years, Jeffrey Sanzel has carefully arranged the order in which the short plays he selects and directs appear on the downstairs second stage at Theatre Three. Often, he tries to leave the audience laughing just before intermission and leave them with something to think about in the final piece. But for this, the 18th annual Festival of One-Act Plays, the finale of the March 8 opening-night  presentation carried special significance, marking a double-zero milestone.

"Bird," an eight-character play by Sarah Pitard, is the 100th world premiere presented by Theatre Three in its critically acclaimed festival. "We're still here," Sanzel says, adding that he figured the festival might last five or six years at best. Seventy-five plays were submitted that first year. Now, no fewer than 400 land on Sanzel's desk annually. The record is 750.

Appearing in the 100th play is the One-Act Festival's most prolific actor. With "Bird," his third of the evening, Brian Smith will have acted in 29 festival shorts. He's also performing in "Clean Routine," a comedy by John Davenport and Jaryl Draper, and "The Present Illusion," by David Vazdauskas, which Smith describes as an "optical illusion for the brain in the manner of an M.C. Escher painting."

"I've played everyone from an alien dressed up as Jesus to a snowman about to get it on with a snow woman," Smith recalls. He's appeared in as many as four one-acts in a single festival, including one with no dialogue. "It's an artistic challenge to go from one character to the next in a matter of minutes," Smith says. One-acts seem to be his specialty -- even when he's played the main stage upstairs, where Smith, a Plainedge Middle School math teacher by day, appeared in David Ives' collection of one-acts, "All in the Timing."

"We try to create a bill where actors don't have to do plays back-to-back," Sanzel says. "But occasionally we're not able to do that. It takes a particularly strong actor to jump literally from not just one character to another but usually a completely different style and tone."

The one-acts are also a "bus-your-own-tables situation," Sanzel says. "We don't hire any backstage crew and there are multiple, often complicated changes between pieces" -- or even within them, as in happy-to-sad "Bird," this year's closer. "You have to be willing to think of this as guerrilla theater -- perform, move furniture, perform, shift props, change costume, perform, rinse, lather, repeat."

Playwrights selected by Sanzel often come back for more. Frank Tangredi of West Babylon holds the festival record with six plays produced. He's been working on a full-length piece and has no entry this year. George Bryjak of Saranac Lake, who writes one-act plays exclusively, has had one in each of the last three years at Theatre Three. He follows up 2013's "Collateral Damage," a story about a soldier returning home from multiple tours of duty, and "Unspeakable," 2014, in which Gypsies try a Nazi in absentia, with "Accident": A husband arrives home, tipsy as usual, with horrible news. He's just fled the scene of a fatal accident.

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"Hopefully," says Bryjak, whose one-acts have been produced from Los Angeles to off Off-Broadway, "I'll have some success with comedies as well at Theatre Three. These have all been pretty heavy."

This year's festival mixes comedy with drama and offbeat fare, including "Quack" by Patrick Gabridge and "Civilized Affairs" by Carl Williams. But Sanzel took a chance two years ago with five out of six plays that dealt with death. "You couldn't find another funny one?" actor Steve McCoy remarked during rehearsals.

"We were so worried that I adjusted the preshow speech to let the audience know what they were in for," Sanzel recalls. "It was one of our most successful and well-received evenings. . . . The festival allows us to go places that are not in our perception of the audience's comfort zone. It's a reminder to never underestimate the audience."

WHAT The 18th Annual Festival of One-Act Plays

WHEN | WHERE 3 p.m. March 14 and on an irregular schedule through April 4, Theatre Three's Ronald F. Peierls Theatre on Second Stage, 412 Main St., Port Jefferson

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