MIDDLEBURY — Take six writers and give them 12 hours to write a 10-minute play. Include some random ingredients, some curveballs, and one line chosen by the public. The next day, grab a director and tons of actors, rehearse those shows and perform them that night at 7:30. That’s “Pop-Up Plays,” popping up for the second year at Town Hall Theater on Saturday, March 7, at 7:30 p.m. The whole event takes place in 24 hours, which makes it the freshest theater experience around.