In the first 18 drafts of her classic comedy, Mary Chase envisioned an actor playing Harvey: a six-foot, or six-foot-three-and-a-half invisible white rabbit. At the Boston tryout, an actor wore a $600 costume, but looked like an actor playing a rabbit. So Chase made Harvey invisible ("more real," she said in an interview). She won a Pulitzer Prize. For audiences who favor subtle, nuanced acting, the Lamb's Players production may require an adjustment. Everyone except David Cochran Heath (splendid as the convivial, egalitarian Elwood P. Dowd) pole vaults their characters over the top. In director Robert Smyth's inventive conception, even though he talks to a "pookah" (an invisible fairy spirit; a "puck" in England), Elwood's the sanest one around. Smyth makes Harvey a comedy not of manners but of neurotics, some of whom begin to grow. As Elwood's sister Veta, for example, Kerry Meads begins WAY out there: a tangle of words and gestures. As the play proceeds and she encounters Harvey's calming aegis, Veta sheds neuroses along the way. As do the others (thanks to fine ensemble work). Mike Buckley's handsome set swivels from scene to scene. Jeanne Reith's mid-'40s costumes and Nathan Peirson's lighting sustain an antic mood. (Note: due to popular demand, Lamb's has extended the show's run.)