Here I'm going to be analyzing the HELLBOY art style, the line-art, colors, shadows, proportions, etc. Plus ways to get similar results.
Lines are thin and have no 'pen pressure' (no thick-to-thin lines).
Many lines are used for texture throughout the drawing which makes things look less flat. It also helps to show distance since things further away have less detail.
This style doesn't use lines to show form, or even for shading.
There aren't perfectly straight lines, or smooth curves. Everything seems hand-drawn, no rulers seem to used.
Most of the colors are very desaturated, except for Hellboy, he stands out from the rest.
There isn't much shading, but the colors aren't flat either. There's a gradual change in the colors, they go from light to dark in a smooth transition.
Colors have a watercolor texture so if you're working digitally make sure to use some watercolor brush or texture.
shadows/ inks are pretty much solid shapes, there's no shading effects like hatching or cross-hatching.
Instead of hatching this style uses some triangular shapes as the edge of some areas.
They're completely black, the lines and inks aren't colored.
Proportions seem to be close to real life proportions but things are changed to be more angular, and more into shapes.
There are not many 'organic' shapes.
I hope this was helpful here's a few Hellboy books if you're interested:
Mike Mignola's Hellboy Artist's Edition (Artist Edition)
Hellboy: The Art of Hellboy
Hellboy, Vol. 1: Seed of Destruction
Hellboy, Vol. 2: Wake the Devil
Hellboy, Vol. 8: Darkness Calls