Jake Gyllenhaal's scary transformation for new film

Jake Gyllenhaal in a scene from his new film Nightcrawler.

US actor Jake Gyllenhaal was scarred filming his latest movie Nightcrawler. Not mentally, but physically.

The 33-year-old actor went through a radical transformation for the character, dropping 13 kilos by barely eating, so he would appear gaunt onscreen.

He also sports a thin scar on the palm of his hand - the result of hitting a mirror during an intense scene on the Toronto Film Festival thriller.

It wasn't actually in the script, Gyllenhaal tells industry bible Variety, adding "it was just a choice in that moment that happened".

It accidentally cut open his hand and Gyllenhaal had to go to hospital to get stitched up, before returning to work, bandaged, some eight hours later.

The shot was so good director Dan Gilroy kept it in the movie, and actually can't imagine the flick without it.

In Nightcrawler, Gyllenhaal plays a paparazzo who follows ambulances so he can record footage of crash victims on his camcorder.

In the in-depth feature with Variety, Gyllenhaal also opens up about pushing himself with his career choices, unconventional preparation on films like Nightcrawler, and playing a boxer on the upcoming Southpaw.

A New Zealand release date for Nightcrawler isn't yet set.

Read Variety's full interview with Jake Gyllenhaal here.

Watch the trailer for Nightcrawler below:

- AAP