In Lillian Hellman’s The Little Foxes, the lives of Southern women don’t look too shabby, but they most certainly have their limits. Regina Giddens (Cynthia Nixon in a recent matinee) and her sister-in-law Birdie Hubbard (Laura Linney—the stars alternate) define the parameters in the superb MTC production at the Samuel J. Friedman Theater, the first being all grins and fangs, the latter, tipsy and gently nuts. Neither one is quite who she seems, however. I am still not over the effect of Laura Linney’s dimpled smile as she instructs Regina’s daughter Alexandra (Francesca Carpanini) in the family’s history, cautioning her to stay clear of her own son Leo (Michael Benz). Cynthia Nixon broadcasts bad faith with the smallest twitch of her mouth, her eyes fixed on a far horizon.