Getting into the right mindset for different roles is an important part of acting. This is exactly what Jared Leto seems to have done during the filming of Masters of the Universe, where, according to director Travis Knight, he reportedly used a variety of methods to surprise his co-stars.
Among other things, Leto reportedly smeared his face with blood-like makeup during filming. Why? Since Skeletor's appearance is created digitally in post-production, Leto had to film his scenes without a mask or anything similar. So this was his way of trying to elicit genuine reactions from his colleagues.
According to Alison Brie, it seems to have worked, and in an interview she said that the blood-like smear was unsettling enough to elicit reactions from many.
Brie tells Screen Rant that:
"He's got the amazing costume on, but it's still just Jared's face because that was all replaced by a CGI skull. But he didn't want the other actors he was sharing scenes with to look at him and see Jared. He wanted them to look at him and see something scary. So he would put this kind of blood-like, smeary makeup on his face. So he looked scary as hell."
Masters of the Universe is currently in theatres for anyone interested.