What's a measured response to this "tease" behavior from a younger woman?

I’m in a play and play a minor character opposite a female actor who is 19 (I’m 40). We began the rehearsal period together and so have had plenty of time to get acquainted. Sometimes we are chatty backstage and sometimes we aren’t. It depends on the day or weekend.

She’s obviously young and is pretty (although not my usual type), and often arrives in appropriately skimpy outfits. In addition, her costume in the first scene is panties, a bra and a slip (or whatever it’s called). It seems like she mostly enjoys prancing around a bit in this costume in the green room. (The cast of this show is mostly middle aged men).

This all being said, she’s not a skanky girl (I would say more so that she’s brazen and maybe an independent mind.) She’s in college and has aspirations to study abroad. She’s also got a boyfriend, and generally comports herself well in conversations, etc. She seems a decent enough girl.

So, normally she does this move in the process of conversing with others where she pulls her knees to her chest and often invites a bit of a peek at her crotch. Whatever. Today, though, we get into a conversation, and while I am sitting normally on a love seat, she sits facing me on the arm of the love seat (with her feet on the butt cushion) and proceeds gradually to push her (long) top between her legs and then spread her legs wide and then wider as she’s talking to me. Her crotch is basically level with my chin and a few feet away, and I’m looking up at her to maintain eye contact. This continued until another woman in the show sat on the stool next to me and facing her at which point she closed her legs, which to me says she was aware of her posturing.

So, while I am or have been a letch in some ways in my life, I’m not in all situations, including this one. At the same time, half the reason I do theatre is to flirt with a cast member, and in this show especially, she’s as good a prospect as any.

So my question is, what’s this behavior of hers about and what’s a good way to respond that creates a bit of a thrill without necessarily taking things too far. What’s a good way to flirt back or what’s a pass I can make at her (assuming it’s warranted) that’s relatively inconsequential. I’m curious to hear a spectrum of responses. Thanks!