and we had always wanted to [with Anna and Elsa] singing in harmony," Robert tells PEOPLE. "So we were kind of salivating. We had just finished Frozen 2, and we were salivating to write one song ...

So you know how we said the teaser for Channing Tatum's Lip Sync Battle - which sees him dressed as Frozen's Elsa singing 'Let It Go' - was the best thing we'd ever laid eyes on?

We know you’ve watched the video for “Let It Go” from “Frozen” a million times, and in it you’ve seen Elsa quickly materializing a snowman that would become Olaf. What we don’t see ...