WEST HOLLYWOOD, Calif. — When John Fawcett and Graeme Manson, the creators of BBC America’s cult series “Orphan Black,” recall the moment they realized the central conceit of their twisty science-fiction conspiracy drama might actually be believable, they point to a bar scene in the third episode. In it, a bespectacled, dreadlocked Ph.D. student named Cosima has a lengthy conversation with Sarah, a street-savvy British grifter.

“They’re looking at themselves in the mirror, and John said to me, ‘It works!’ ” Mr. Manson recalled, referring to the fact that both Cosima and Sarah — as well as several other characters from around the world who discover they’re clones — are played by a single actress, Tatiana Maslany.

“Even after we cast Tat, we knew we had the right person, but we were really worried,” Mr. Manson said. Their concerns stretched far beyond the fact that if the series was going to succeed, Ms. Maslany had to be able to make each of the clones seem like a completely different person.

“One of my biggest fears — and theirs, too — was ‘Will people buy me as not 16?’ ” said Ms. Maslany, who is 28 but has been acting professionally since age 9 and was still being cast as troubled teenagers until just a few years ago. Having never landed an adult role, she was now being asked to play a grown-up times seven, and tap into a different skill set.