Playwright David Greig had two reactions when he was asked to adapt Dr. Seuss’ “The Lorax” or the stage.

First reaction: He jumped in the air and clicked his heels together.

Second: “Omigod. How in heaven’s name am I going to do that?”

Certain books in Children’s Theatre Company history seem like naturals. Of course it’s possible to make a treasured tale such as “Ramona the Pest” into a play, or “Akeelah and the Bee.” But picture books like “Harold and the Purple Crayon” or “The Snowy Day” — which, not to put too fine a point on it, have no characters or plots — make for bigger challenges. Nevertheless, CTC persists. In fact, both shows now playing there, “Corduroy,” which runs through May 20, and “The Lorax,” currently in previews and opening this weekend, are in that tricky-to-adapt vein.

“I remember reading ‘Corduroy’ to my own children many times and I’ve clocked it at five minutes,” says Barry Kornhauser, who wrote the play for CTC. “Turning that into a full-length piece is daunting.”

“The brilliant thing about these books is they leave you lots of space,” says Greig, accentuating the positive.

Seuss’ book has been a favorite since its publication in 1971. Appearing just a few months after the first Earth Day and the establishment of the Environmental Protection Agency, it is beloved not just for Seuss’ signature wordplay but also for bringing the ecology movement to children.

Although the Lorax is the title character, Greig was drawn to the book’s villain.

“The thing that struck me immediately as a dramatist is that Once-ler is, in some ways, the baddie of the book but he’s also very human. He’s us,” says Greig. “I love flawed characters and that’s what’s so attractive about the Once-ler. He’s a very enthusiastic guy on an adventure, who just wants to feed his family and explore stuff.”

Once-ler (Steven Epp) is an inventor who uses foliage tufts from Truffula trees to knit useless items he calls thneeds. They become so popular that Once-ler depletes the resource, which is decried by the Lorax (a puppet operated by H. Adam Harris, Meghan Kreidler and Rick Miller, with Harris supplying the voice), who insists, “I speak for the trees.”

“It does have an environmental message but it’s not simplistic or preachy or hand-slappy,” says Greig of the play, produced in partnership with London’s Old Vic and San Diego’s Old Globe, where it will play after CTC. “I saw the challenge as really interesting: to find in Once-ler both a hero and a villain, someone we could really identify with.”

Greig, whose “The Events” was staged at the Guthrie Theater in 2015, challenged himself to reread all 60 of Seuss’ books so the distinctive rhymes and made-up words became almost a part of his dramatic DNA. As he puts it, he “Vulcan mind-melded” with the late, legendary wordsmith.

Kornhauser also researched his book’s writer, Don Freeman, to make sure the play’s embellishments — beefing up the slapstick role of a night watchman and creating a parallel story line for Lisa, who longs for the titular stuffed teddy bear but who is only briefly depicted in the book — conveyed “Corduroy’s” sweet spirit. The research revealed that Freeman had been interested in the idea of a deserted department store at night, something the play elaborates on.

“You’re striving to make it work in a different medium but you want to be true to the core values of the original,” says Kornhauser, who cranked out at least 15 “Corduroy” drafts, with input from CTC’s artistic director, Peter Brosius, and its former director of new play development, Elissa Adams. Kornhauser says it also helped to know that gifted physical comedians Dean Holt and Reed Sigmund would be doing “Corduroy,” allowing the play — like the book — to exist with very little dialogue.

In addition to the theater folk, Greig says he had advice from the estate of Dr. Seuss, which pointed out anything that didn’t strike them as Seuss-y enough. Greig figured he’d nailed it when he came to this realization: “There are some things in the play that, now, I can’t remember if they’re from the original book or me.”