Starlight’s battle in The Boys season 4 represented a big acting challenge. So Erin Moriarty turned to one of the greats.

Following in the footsteps of the baddie known as “Doppelganger, “it was only a matter of time before Prime Video‘s superhero satire The Boys introduced another shapeshifting character. That moment finally arrived in season 4’s penultimate episode “The Insider,” in which a chameleonic assassin took on the form of Annie January a.k.a. Starlight (Erin Moriarty).

In the finale,* the shapeshifter is thwarted in their mission to take down U.S. President-elect Robert Singer (Jim Beaver) by none other than the real Annie January herself. To put it mildly, embodying two characters involved in the same physical fight would is a bit of an acting challenge. Thankfully, Starlight actress Erin Moriarty knew exactly where to turn for inspiration.

*Following the attempted assassination of former President Donald Trump on July 13, Prime Video removed the original title of this episode, “Assassination Run,” and added a disclaimer that reads, in part: “The Boys is a fictitious series that was filmed in 2023, and any scene or plotline similarities to these real-world events are coincidental and unintentional. Amazon, Sony Pictures Television and the producers of The Boys reject, in the strongest terms, real-world violence of any kind.”

“I remember watching The Dark Knight with Heath Ledger. I needed to see someone transcend their physicality in front of my eyes to believe like that I could do it,” Moriary told Den of Geek prior to The Boys season 4 premiere.

At first, Heath Ledger’s portrayal of The Joker in Christopher Nolan’s epic 2008 film might seem like a bit of an odd fit in comparison to Moriarty’s dual roles in this finale. After all, there is no wanting for examples of the same actor playing multiple roles in a project from Nicolas Cage in Adaptation to Peter Sellers in Dr. Strangelove to even this year with Robert Downey Jr. in HBO’s The Sympathizer. According to Moriarty, however, looking to Ledger in The Dark Knight wasn’t only a matter of discerning technique, but gaining confidence that a certain level of acting is even possible.

“I needed to transcend more than anything,” she said. “It’s one thing to change your voice. It’s another to change your intentions and physicality. So I studied the Alexander Technique, which is like trying to use animals to inspire your body. That’s what Heath Ledger used. And I had to literally see an example of it in front of my eyes to believe I can do it.”

The Boys season 4 put Annie January through the wringer. In a disturbing bit of real life allegory, Annie becomes the target of a QAnon-style conspiracy movement that accuses her of being the queenpin of a child-trafficking cabal. Involved in this movement is Firecracker (Valorie Curry), an unwelcome blast from Annie’s past to remind her that she hasn’t always been the heroic figure she’s wanted herself to be.

To better match the intensity of Annie’s arc, Moriarty researched dissociative identity disorder and tried smelling salts before filming scenes. Still, even with all of Starlight’s struggles this season, the final battle against herself remained the biggest hurdle

“It was the most satisfying, challenging experience I’ve ever gone through. I didn’t know how it would hit but I needed to go there.”