FORT WORTH — For Diana Sheehan, it is the Hart of the matter that matters.The Dallas-based songstress makes a major case for Richard Rodgers’ first important collaborator (long before Oscar Hammerstein came along) in her revue Nobody’s Hart: Diana Sheehan Sings the Lyrics of Lorenz Hart, which opened a four-performance run at Amphibian Stage Productions on Thursday night.With the theater set in a mix of cabaret tables and traditional seating, Sheehan delivers Hart tunes both familiar and rare with only the spare, but perfectly modulated, support of pianist James McQuillen and cellist Sarah Choi. And in between numbers, Sheehan provides entertaining and insightful descriptions of the lyricist she calls “Larry” (and, considering how well she knows him, she should). Sheehan’s set list offered such favorites as My Funny Valentine, Bewitched, Bothered and Bewildered and I Wish I Were in Love Again between obscure but also engaging tunes from forgotten shows such as Garrick Gaieties and Peggy-Ann.That is the problem that Hart, who died in 1943, faces in our times. His work with Rodgers has been supplanted by the later, and justly legendary, work that composer did with Hammerstein. But Sheehan makes it clear that, although the musicals he wrote may now be out of fashion, the superior quality of his lyrics never goes out of style. Sheehan’s performance, delivered in a gorgeous, floor-length gown suitable to the period of most of the music, is notable as much for what she doesn’t do as for what she does. She has a splendid, well-schooled voice, and is perfectly capable of wowing an audience with big notes. But since that is not what these Rodgers and Hart songs are about, she holds that aspect of her talents in careful check. She concentrates on the phrasing and the shaping of the lyrics, with wonderful effect. Even on the numbers we know well, she makes you feel like you are truly hearing the words for the first time. And McQuillen and Choi provide exactly the support she needs without ever calling undue attention to themselves.This is obviously a different kind of entertainment not intended for the Justin Bieber crowd. But if you are a fan of American musical theater, you are likely to leave Sheehan’s show with a new appreciation for Hart’s compositions, as well as a greater understanding of who the man behind the words was.