With a schoolboy’s enthusiasm — indeed, looking a bit like Harry Potter all grown up — the writer Greg Pierce flipped through the three-ring binder on his lap, his forehead crinkling as he searched for the morning’s to-do list. He was sitting in the Upper West Side studio of John Kander, the Tony Award-winning composer of “Cabaret,” “Chicago” and more than 15 other musicals over the last 50 years. Show posters of all that fame hung on the room’s west wall, most of them featuring the legendary names Kander & Ebb — the latter, of course, being the lyricist Fred Ebb, who died in 2004.

Mr. Pierce, who is 34, is now the 85-year-old Mr. Kander’s new collaborator — more earnest and less deadpan than Ebb, but just as determined and, lately, just as busy.

Throughout May Mr. Pierce has been juggling their musical, “The Landing,” a triptych about love and obsession that is now running Off Broadway, with rehearsals for “Slowgirl,” his tense new play about a troubled young woman visiting her uncle in Costa Rica. Both projects are attention-grabbers: “Slowgirl” is next month’s inaugural production at Lincoln Center Theater’s new stage, the Claire Tow, while “The Landing” is no less than Mr. Pierce’s debut as lyricist and book writer at the side of a musical theater legend.

There he was, quietly watching as Mr. Kander conjured from his keyboard a melody for an interlude in “The Landing.” When a note went amiss, Mr. Kander banged his palms on the keys and swore. Mr. Pierce’s body language, except for the occasional scratch of the chin or lower back, conveyed pure concentration.