The Blindspot actress and the Supergirl villain got engaged in March but are in no hurry to get hitched.
"We're just enjoying, having a good time, and we take things slow," Alexander said at the Television Critics Association party at the Spago Beverly Hills in August. "We just enjoy each other. A piece of paper isn't going to change how much I love him, and how much he loves me."
In May the actress, 31, also talked about their hectic lives, telling PEOPLE: "We're both very busy people, and we want to make sure that when we get married, we actually have the right amount of time to spend with family and friends because it's gonna be such a fun celebration."
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Alexander's been weight training and working on her flexibility nearly non-stop for her upcoming projects, including NBC's inked-up whodunnit Blindspot, and she has the bruises to prove it.
"When I play Lady Sif in the Thor films, it's very fancy moves. It's very glitz and glam in a way," says Alexander. "[But Jane Doe in Blindspot] is hand-to-hand combat, very realistic moves for my body weight and size against the opponent. And it's a lot more rough. I'm taking much more of a beating on this than I ever did in any Thor films."
Alexander does her own stunts. She says she and her stunt double of 10 years, Ky Furneaux, sometimes tag team it "because we get beaten up pretty bad".
Despite her tough exterior, Alexander has a real soft spot for her husband-to-be, who she says is "the best."
Facinelli, 41, has also been getting ready for his role as the evil Max Lord in Supergirl. While she told PEOPLE the couple's newly blended, good-and-evil household dynamic is "intense," it appears the whole family is looking forward to what's in store.
"The kids are super excited about [Supergirl]," Alexander says of Facinelli's three daughters – Luca Bella, 18, Lola Ray, 12, and Fiona Eve, 8 – with ex-wife Jennie Garth. "They can watch that show. They can't watch mine."
Reporting by SCOTT HUVER