Mostly reference guides, like how a foot rests on a high wire, or what a camel looks like.
I do a lot of research, because now I'm developing my new style of drawing. I look at other people's illustrations or daily things that can inspire me to draw.
I always make sure to do visual and background research on the subject matter I am illustrating. I like to make sure the things and people I depict are accurate not only in visuality but also factually.
I always research photographs and drawings of the animals I'm drawing. Sometimes I end up reading about them and finding interesting things that I might include in other work.
I'll do whatever research is required, whether it's for a photo realistic likeness or a costume design.
Anything that's needed depending on the project. If I am designing a character I may research other character designs of artists I admire, I will research style to find one appropriate to the project and most importantly I will research any gaps in my knowledge, whether it is costume design, historical background, anatomy, or cultural context.
Quite a bit usually I am a perfectionist, especially if its a period piece from a certain era.
It depends totally on the assignment, but I always try to be as accurate as possible. Basically, when I do infographics, I may have to do the whole research myself, which is no problem since I write articles occasionally too.
I make sure to keep up on my art history. It's important to know what has been done and how it's been done. If I like an artist, I'll look at who they're looking at.
Because I carry various styles on my portfolio, it's one thing I first ask to my client.
Later on, I will research everything that is out of my knowledge to draw, right now it would include the perfect anatomy of a vehicle, a word I don't know the meaning of, certain cultures (costumes, food, colors, etc) of the world, etc.
I google images on the subject and do some free drawing as a form of brainstorming.
I use the library and Google.
That depends on the complexity of the project. It may require that I read a particular book on the subject, or seek expert input on how best to convey an idea.