Although the sky looked stormy at Thursday’s Central Park opening of “Into the Woods,” it never rained. But skyrocketing humidity ensured it was plenty moist.Especially for the actors in the Stephen Sondheim tuner, presented as part of the 50th anniversary season of the Public Theaters’ Shakespeare in the Park program at the Delacorte. Many of the poor thesps in the cast — which includes Amy Adams, Donna Murphy and Denis O’Hare — are stuck wearing wool sweaters, tweed blazers or heavy wigs. They all looked pretty comfortable on opening night, but chalk it up to the magic of the theater. “It’s incredibly hot,” said O’Hare, on his way up to the post-show shindig at the park’s Belvedere Castle. “I’ve lost 2 pounds doing this role.” But everybody looked so cool and dry! “I have one word to say to that: Bounty,” O’Hare added.