Yes, it's a film version of the Shakespeare play, direction and screenplay by The Man. I got to a preview showing last night; it opens this weekend. If you were expecting Hero to come back from the dead as a vampire or something, forget it, OK? This is not that Joss Whedon.
It's just a straight version of the play, filmed in black and white, which emphasizes that watching it feels like taking a long sip of cool, dry wine (in which you're joined by most of the characters). There's only a few notable changes.
First, modern setting. Except for the interrogation scene in a police station, the entire movie is set at a wealthy suburban home in Spanish colonial style and on its extensive grounds. The men are usually wearing contemporary suits with narrow ties (including the cops: I guess they're supposed to all be detectives); the women, mostly summer dresses or maids' uniforms, depending.
Conrade is a woman (and Don John's lover); the script is uncertain whether to adjust the pronouns to acknowledge this. And a couple of brief scenes seem to be flashbacks saying that Beatrice and Benedick had a past romantic relationship; this would be a catastrophic misunderstanding of the characters and is best ignored.
Never mind all that. Mostly, this movie is a performance of the play emphasizing the light comedy (I've seen stage productions treating it as a near-tragedy), with a cast mostly filled by established members of the Joss Whedon repertoire company, with everybody giving the most easy, naturalistic, non-declamatory readings of their lines imaginable. That's the real achievement of this movie, one which the trailers don't adequately convey. Nathan Fillion, entirely in his element as Dogberry, is the most amazing, as good as any I've ever seen, but they're all terrific.
Of the other actors whose work I know, the most like what you'd expect are Tom Lenk as Verges (Dogberry's assistant) and Sean Maher (though lacking the big black mustache that his Simon Tam once said he'd grow on establishing himself as a villain) as Don John. Alexis Denisof plays a Benedick who's entirely the seasoned Angel Wesley and not a touch of the awkward Buffy Wesley, even when the character is at his clumsiest or most foolish. Amy Acker as Beatrice and Fran Kranz as Claudio are playing characters entirely unlike their Whedon TV series roles, and accordingly act entirely differently. Acker omits the burning rage of most stage Beatrices; instead, she's sly and piercing. Kranz, who has to be shy rather than nerdy, is at first almost unrecognizable.
And little touches of physical humor make the play. The sexton, incarnated as a female police stenographer, rolling her eyes at Dogberry's attempts to give dictation; Benedick and Beatrice each cavorting in astonishment as they eavesdrop on the testimonies of their loves; Leonato and Don Pedro calming Claudio down as he gives an implausibly overheated description of Beatrice pining; Dogberry and Verges finding they've locked themselves out of their car when leaving at the end; and the moment that will probably make this movie immortal in cinematic history, though it goes by so fast you could blink and miss it, The Scene With The Cupcake.