Not that Plimpton is doing Midsummer for the heck of it. Of all the young lovers stumbling through the woods near Athens being juiced and goosed by Puck, Helena is the scene-stealer as the wailing, supposedly homely one who finds herself suddenly the object of both Demetrius’s and Lysander’s desire. The role fits into the niche Plimpton has carved out for herself in the six years since she moved from Chicago, where she did acclaimed work at the Steppenwolf Theater, to New York. Here, in diverse works such as Hobson’s Choice, Sixteen Wounded and Shining City, Plimpton has transmuted her saucy teen persona into a figure of regal bearing, larger-than-life humor and a flair for dense language. Like Billy Crudup, Philip Seymour Hoffman, Elizabeth Marvel and Dennis O’Hare, she’s one of those rare stage creatures whose gutsy, flamboyant performances work best live.

One person who has watched Plimpton’s theatrical maturation is Sullivan, who directed her in Seattle Repertory Theatre’s 1994 production of The Sisters Rosensweig. “Martha’s always been a wonderful, instinctive actress who has accumulated more technique over the years,” he says. “After coming out of Lincoln Center, she got her energies back and was ready to go. I was a little surprised—and delighted—that she immediately wanted to get on board.”

Sullivan’s Midsummer will have a spare, elemental quality, with a set dominated by a huge tree designed by Eugene Lee (Wicked) and characters in Victorian dress. Since Utopia spanned 1833 to 1866, Plimpton’s new costuming won’t be radically different from that of her last gig, but the language will. And she has only one other Shakespeare on her résumé, a 1991 Pericles—at the Public, naturally. Has immersion in Stoppard’s rhetorical cascades made it easier to mouth the Bard? “It’s hard to say, ‘Well, of course it feels perfectly natural to go from Stoppard to Shakespeare,’ ” Plimpton muses. “That would be asinine. It’s all difficult, and you just sort of go at it bit by bit, hoping you tell the story clearly.”

She doesn’t stint with her homework. On Plimpton’s charming MySpace page (a rare and candid undertaking for a celebrity), she posted a marked-up First Folio page from Midsummer. But when the website is mentioned, she practically blushes over the phone. “It’s just a MySpace page, for God’s sake,” the actor says. “It’s something I do to have a good time and communicate with friends and colleagues and be silly.” Still, the page—which features photos of role models such as Gena Rowlands, Anna Magnani and Gilda Radner—has a practical use: Plimpton’s writing about her favorite bands got her work reviewing music on MTV’s website.

Music critic, blogger, stage and screen presence: Plimpton might be accused of being a workaholic. After Midsummer winds down in early September, though, she intends to have real downtime. “My immediate plan after we close is to go to Puerto Rico and take some fucking vacation,” Plimpton says with a laugh. “Because my butt is getting tired.”

A Midsummer Night’s Dream is playing at the Delacorte Theater. See “Shakespeare alfresco.”