Sabrina Carpenter (who plays young Merrin) on the set of Horns. She has it posted on her website. Below also another one which was released online earlier.
Update: 19th February. A French video from Cinema from Canal+ shows more behind the scenes footage.
Edit: The video seems to be removed again by Canal+ so I also added an YouTube upload
Peter Guber (Mandalay Entertainment Group CEO) I re-tweeted this one on Twitter before.