Mea culpa. I should know better. Having seen (and been seriously underwhelmed by) Jeff Bowen and Hunter Bell’s [title of show] off-Broadway (with, let’s note, Jeff Bowen and Hunter Bell playing the leads in what is essentially a musical-à-clef about two theater queens writing a musical-à-clef), I was in no rush to catch it at Signature Theater. Hence this three-weeks-late revie….um…rave. Silly me. Signature’s musical rep has all to do with polishing to gemlike clarity shows that have failed to gleam elsewhere. I should’ve known they’d do it again. Happily, audiences appear to be flocking to [title of show]. The place was packed, and rolling with laughter (as was I) at the very jokes that had seemed so wan in New York. The absence of Bowen and Bell (whose presence seemed the whole reason for the piece’s existence at the Vineyard Theater) turns out not to be a drawback. They’re passable performers, but their Signature avatars—Sam Ludwig (as Jeff) and James Gardiner (as Hunter)—are knockouts, earnestly goofy and effortlessly uproarious as they pen a quickie musical to compete in the NY Musical Theater Fest, then rejigger it for a run in Greenwich Village, and then rejigger it again for Broadway. All that rejiggering’s necessary because their musical is about writing their musical (“Just put this exact conversation in the show”…“Wait, so everything I say from now on could actually be in….?”), a Pirandellian notion from which spring songs called “Filling Out the Form” (the inspiration for the show’s bracketed title), “I Am Playing Me” and “Untitled Opening Number.” Ludwig, who was a fine Toby in the troupe’s Sweeney Todd a few weeks back, has a neatly understated Michael Cera thing going as the meeker of the songwriters—quizzical eyes, slow-burn smile, knees and elbows tucked-in tight when he’s pressed into dancing. James Gardiner is his polar opposite—a wide-eyed, grinning, musical comedy wild-child bouncing every second as if the balls of his feet have been rubberized. Providing distaff distraction as the real-life performer-friends who came along for the ride are Erin Driscoll (seemingly channeling Reese Witherspoon) and Jenna Sokolowski (dripping sarcasm in the manner of a 20-something Bea Arthur). Together they’re not merely as amusing and vibrant as latter-day Mickeys and Judys putting on a show; They manage to make a tightly choreographed (courtesy of director Matthew Gardiner) and brightly musical (keyboarding by Gabriel Mangiante) evening seem largely improvised.