In its rehearsal spaces and on its main stage, the venerated Walnut Street Theatre currently resembles a jungle gymlike maze of elevated catwalks and open portals - all for Peter and the Starcatcher, which opens Thursday.

Peter is such a massive, madly active production that, for the cast to rehearse adequately, a replica of the huge, multilevel, Rube Goldbergian set on the main stage, all ladders, paths, and openings, had to be built in the fourth-floor rehearsal space, for which, in the words of director Bill Van Horn, "rail heights and step lengths were cut in half, to get used to the real estate." Sunday's dress/tech rehearsal, at which "actors must rediscover new places to swing and lean out," was the first time the cast moved from the miniature set in the rehearsal space to the main-stage structure.

Peter and the Starcatcher is Rick Elice's play based on the 2006 novel by humorist Dave Barry and Ridley Pearson. A prequel to J.M. Barrie's Peter and Wendy, both book and play provide a mischievous backstory for that ultimate man-child, Peter Pan.

Thirteen top-notch improvisational actors - "all Philadelphians," says Van Horn - play 100 roles, jumping and moving in serpentine fashion. Within this magical world, a dusty attic becomes two warring battleships, wood-framed paintings become doorways, cats can fly, and billowing skirts become sails perfect for a stormy, seafaring voyage.

Van Horn, set designer Todd Ivins, lighting director J. Dominic Chacon, costume designer Mary Folino, assistant director/actor Aaron Cromie - with a stellar cast of Philly thespians, including actor/music director Alex Bechtel - all worked together to create Peter's world.

"It's wood and steel, nothing out of the ordinary," Ivins says of the main set. The back wall was computer-routed before being painted: "What looks like a complex series of overlapping slats and beams," Ivins says, "is actually completely flat." It all comes down to ergonomics of stage design, traffic patterns, railing heights, matters the public rarely hears about. Ultimately, his staging for Peter is, in Ivins' words, "just theater magic."

Typically, Ivins says, at the Walnut, there are not many levels on the stage - multilevel stages are rare, in general, for regional theater spaces. "I happen, though, to be very interested in levels, for the rehearsal space and the stage," he says. "The muscle memory needs to rehearse that, have a place to get that into your bones." So Ivins and Van Horn created a half-size version of the finished stage. "Peter's so interactive and improvisational, we wanted every opportunity for them to explore all that on a miniaturized version," he says, "so we could think and dream through the process."

One aim was the impression of spontaneity on stage - to make the audience feel as though Peter's staging, choreography, and patter are "made up as we go along." Just as the Marx Brothers did with playwright George S. Kauffman's The Cocoanuts and Animal Crackers on stage, every spontaneous moment was worked out in advance to the last pirouette. "It's organic, flowing, with every actor grabbing things on stage," Ivins says, "from a new coat or hat to a pole or a picture frame - and turning into their scene."

Van Horn credits democratic collaboration among actors and crew from the start. Much came from hiring Cromie, not just to act, but also to be a second set of eyes. He was brought on as Smee - one of the more musical buccaneers under menacing-but-flowery pirate Black Stache (Ian Merrill Peakes). "You can't cast Aaron and not rely on his mastery of puppetry, special effects, musical instrumentation, and his craft as a creative director," Van Horn says. "He's a resource, a part of a collective enterprise."

Cromie is a director and mask- and puppet-maker renowned for dozens of local theatricals, including Fringe Arts' The Body Lautrec of 2014 and Shakespeare's Titus Andronicus at the Philadelphia Shakespeare in 2012. "As a multidisciplinary artist," he says, "I just became another set of eyes for people. It lent to the show's collective feel." Cromie says acting with comic thespians such as Lindsay Smiling and Dave Jadico is like "hanging out with guys who like each other while getting a chance to play with big toys."

"We anchored the show in the reality of an attic, an industrial-sized attic, but it's like the playwright says at the beginning of the script: We want you to dream with us," says Ivins. "The attic can become anything at any moment. One character can become another in a split section, with any actor taking on the story's narration. We just provide the dreamscape and ask the audience to come with us."