His other co-star Rebecca Ferguson.
Fans have come to the conclusion that the pair are actually dating IRL, after Zac's really cute social media activity.
It all started when Zac shared a photo of himself and Rebecca on set of the movie, however Rebecca is staring lovingly at Zac, he is smiling at the camera.
He wrote, "A pic is worth a thousand words. Only ones I can think of now are: wtf were u thinking dude?! Look at her!! #GreatestShowman #RebeccaFerguson."
Although a week later, Zac uploaded another photo, this time reversing the roles.
"I finally looked... #GreatestShowman."
OH my god, CUTE!
Fans obviously were living for it.
And honestly, same.