The "Titanic" star thought it would be fun to lock lips with her castmate as part of her celebrations and scheduled the intimate scene with filmmakers accordingly.
"I just wanted her to have, frankly, a great memory in her film life, regardless of how the scene played out or the movie turned out...," Kate told Entertainment Weekly. "I knew that it would be (great) just because of the experience that we would share together."
Kate previously admitted she was "by far the least self-conscious" she's ever been filming a sex scene when she shot one with Saoirse in the period film, stating, "Saoirse and I choreographed the scene ourselves... I just think Saoirse and I, we just felt really safe. (Director) Francis (Lee) was naturally very nervous and I just said to him, 'Listen, let us work it out'. And we did: 'We'll start here. We'll do this with the kissing, boobs, you go down there, then you do this, then you climb up here'. We marked out the beats of the scene so that we were anchored in something that just supported the narrative."
Kate previously revealed that she tended to be protective to her other younger co-stars. Back in September, she told Vanity Fair that she once stayed on "Mare of Easttown" set late to ensure that Angourie Rice, who was 19 at the time, was safe while filming an intimate scene.
"I actually ended up being in the trunk of the car, screwed up in a little ball, just so they weren't alone in the car with two men who, by the way, are completely lovely, respectful, seasoned camera operators," she recalled. "But still, she was young and it was a potential trigger moment for her. I didn't want for her to feel that way."