In its third season, “Supergirl” has proven the value in getting to know who a character is and what might make them become a villain.

Rather than having having a powerful new threat just pop up in National City, the season has devoted nearly as much screen time chronicling Samantha “Sam” Arias’ (Odette Annable) journey to becoming the Worldkiller Reign as Supergirl (Melissa Benoist) gets herself.

“From the very beginning, I said even at Comic-Con, this is the first time we’re seeing the origin of the villain,” Annable tells Variety. “I didn’t just show up and start killing people, and that isn’t even what Reign is about. She’s not coming in to go on a rampage. She’s very specific, and dispenses her justice in a way that she sees fit. I think that the slow burn, and seeing Sam’s life and how they wove her into the lives of other characters on the show makes for a great story.”

Sam doesn’t know that she is Reign, but she has realized that there’s something wrong with her. Here, Annable teases the two sides to her character, who might find out the truth about Sam before she even does, and how close she is to domination.

Has what initially interested you about playing Reign changed over the course of the season?

It hasn’t actually. The reason why I signed on to the show and was excited to play these characters is because of how much the writers are giving me to work with. Both characters are so dynamic in different ways. I get to explore so much humanity with Sam and what it means to be a mother and to protect what she’s built. And with Reign, she’s just so powerful and so fierce and so fun.

There’s a very fun Jekyll and Hyde quality to Sam and Reign.

Yeah, that’s what I love about it. Sam has no idea that she’s Reign, and it’s so heartbreaking to see her desperation and seeing that unfold throughout the season. It’s really dynamic.

Is it ever difficult to hop back and forth between the headspace of the two characters?

I thought it was going to be difficult to jump between the two, but in my mind they’re not connected at all. One of them is one thing and the other is something completely different so I’ve separated them completely in my head that they don’t bleed into each other at all.

We’ve seen a bit of Sam’s humanity bleed over into Reign. Can you tease anything about their internal conflict in the episodes to come?

I will say that there is a turn, and you’ll see it very soon. It has to do with Sam exploring and really digging into what’s going on with her body.

Sam finally confided in Alex that something is wrong with her. Are the two of them going to continue working on figuring out what’s wrong with her alone or is she going to loop in Kara and Lena?

Right now Sam’s in a place of desperation. She’s going to turn to whoever she can to find answers. She’s on a mission to find out what’s going on and she’ll enlist whoever she needs.

Ruby has to be catching on that there is something wrong with her mom.

Yeah, Ruby is a really smart cookie. Kids are intuitive, and Ruby knows that something’s going on with her mother. Sam is fiercely trying to protect her daughter, and that also comes to a head.

Another Worldkiller, Purity, is making her appearance in this week’s episode. How does Reign factor in with her?

I don’t think that Reign feels threatened at all by Purity. When she sees her it’s a moment of “There’s my sister, there’s my ally.” She can train her, and this completes her. Reign knows that finding Purity is one step closer to domination.

“Supergirl” airs Mondays at 8 p.m. on the CW.