Sarah Drew and Justin Bruening Reunite for a Steamy Christmas Movie: 'There's a Lot of Kissing!'

This Grey's Anatomy reunion has some spark!

Sarah Drew and Justin Bruening — whose Grey's characters April Kepner and Matthew Taylor were married and recently divorced — are costarring in a Christmas movie together: Reindeer Games Homecoming, premiering this weekend on Lifetime, which Drew also wrote.

But April and Matthew would blush if they could see the heat between Mac — a former resident in a Boston-based hospital who returned to her small town just before her dad died and is now teaching science at the local high school — and Chase, her high school lab partner and crush, now a Hollywood movie star struggling to be taken seriously.

Drew and Bruening credit their history with helping to create their easy chemistry.

"We've been working together for 10 years," Drew explains to PEOPLE. "It's so easy to just leap back into that mode with him and we have this second-hand. There's no kind of trying to figure out the chemistry. It's just there."

That chemistry is also evident in their easy banter as they discuss Reindeer Games Homecoming, how it defies several typical holiday movie tropes, and their history, below.

PEOPLE: Sarah, when you were writing this movie, did you picture Justin in the role?

SARAH DREW: Actually, I was writing it with some other people in mind after having just worked with a group of people that I wanted to work with again. And then all kinds of things wound up happening over the three years between when that all happened. And then when Justin's name was brought up in our leadership meetings, I was like, "Yeah." I'm so glad he ended up doing it.

JUSTIN BRUENING: Yeah, basically they all said no. And then I was like— That's what it was. Just kidding.

DREW: That's not true. But he and I have worked together so many times, so it wasn't my initial thought. I mean, I love working with you, Justin. But you know, also want to work with other people too. But when his name was brought up, I was like, "Oh my gosh, this is so perfect. Yes." I just can't quit you, Justin.

BRUENING: I'm like a Lay's potato chip.

So where did the idea for this movie come about?

DREW: My husband went to Dartmouth College and every winter they do something called the Winter Carnival, which is a big, whole school competition. They do the polar bear plunge, except in at Dartmouth they cut a hole in the frozen-over pond and leap into the hole. They have a human dog sled derby, but it's on snow and it's people dressed as clowns pulling someone in a sled. I just thought it's such a fun, community bonding sort of experience, and I was like, man, this would be a perfect setting for a holiday movie if it's a small town who always really gets into it.

And then Mac is she's really based on my mom. My mom is a brilliant scientist and she was my biology and AP biology teacher. She has her Ph.D., she did science research for a really long time. She's unbelievably smart, so passionate about kids and about science. And then my actual dad played my dad. So he did the voiceover for the letter and then he's in all the pictures. So I got to honor my parents in it as well.

This movie defies some usual Christmas movie conventions, especially that typical mid-movie almost-kiss. Mac and Chase go for it outside her house! And then, gasp, did they maybe even go inside?

DREW: There was the line [said by Mac's friend and neighbor Simon, played by Brian Sills], "I'm feeling more like a you-got-lucky latte." There's a little bit of extra meaning in there. We're just leaving it to the imagination. Maybe somebody got a little lucky that night. I don't know.

BRUENING: There is no proof. You didn't see me go out the window in the way back and do a total walk of shame.

Justin, when you saw that in the script, were you surprised by the heat between Mac and Chase?

BRUENING: No, that was the great thing about reading her script. I was joking with her at the beginning: You know you get that phone call from your friend who's like, "Hey, so I wrote a script," and right away you're kind of like, "Huh, really? Great." But it was so funny and, having worked together so many times, I know how collaborative she is and how she does want to elevate everything. But I was like, "Wait, we broke up? That doesn't happen in those movies. They don't get together fully and then break up." And then I was like, "I kissed her halfway through. That's not normal." There's a lot of kissing in this movie! It's only supposed to be one.

DREW: I wanted to create a relationship that felt really true and really honest and didn't scratch the surface, but had legs to it and had roots to it. And I think it was super beneficial getting to work with Justin since we've been working together for 10 years. We're just able to immediately leap in and there's no kind of trying to figure out the chemistry. We have a decade of history. I've watched him have three of his children, not in person, but I knew him before.

BRUENING: Yeah, it was awkward. She delivered two of them. It was so strange.

DREW: It was so weird. I caught the baby. But when we started working together, you were about to have Zane, who's your second now. Now you've got four kids. We just have a little history. And I think it adds to the authenticity of Mac and Chase's history.

What was the most fun scene to shoot?

BRUENING: They were all fun. The stage was fun, when you're picking costumes [for the sled derby].

DREW: Yeah, that was probably my favorite scene, actually. It was our last scene we shot of the whole movie. It was up on the stage in the theater. It just felt really fun and really nostalgic and really theater kid geeky and awkward and romantic and a little sexy and funny. I mean, it had just all those little things that sort of came alive, but that was a lot of fun, that scene.

The derby cars: Did they actually fall apart unintentionally?

BRUENING: The one that I pulled apart was not designed to come apart. I took the handle off.

Was the choice to have Mac's residency in Boston an intentional wink to where April currently wound up with Jackson on Grey's?

DREW: No, actually that was written three years ago. So it had absolutely nothing to do with it. But I mean, if people want to take that as a wink, that's fun. I just thought that that's a good place to go to be smart. Boston feels like a smarty-pants kind of town.

BRUENING: It's wicked smaht.

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Reindeer Games Homecoming premieres Saturday at 8 p.m. ET on Lifetime.