I have a quick question to for many of you artists out there, when creating a new idea, like a character, vehicle, weapon, etc. Do you get your idea by making it up as you go along, or do many of you start drawing with the idea already in your mind?

A mix of both - usually I'll know vaguely what I want because it's described in the story or needs to express some particular personality or elemental alignment, etc. Then I'll go gather relevant source images from google image search or art books, and sketch the structural elements that particularly interest me onto a doodle sheet, along with any I have imagined. Then if I know what I want I'll start actually drawing it, or if I still don't know what I want I'll do various experimental sketches until I find an idea I like, and then draw that.

i usually just know vaguelly what i am supoposed to make, lookup relevant reference images and then have a really clear idea in my head and model that. it depends on how good you're at visuallizing stuff i guess, but if you're not a very visual person in your mind you might want to go with sketching it out.

ALWAYS draw out complicated or unique/imaginary models before you actually model them... I never used to do this, but once I tried it and learned how to draw properly, I was ashamed to say I thought I was 'talented enough' to get away without concepts.
No matter how clear you have something in your mind, its not as clear as you think, and once you start making the model its a bad time to go and change things.

I usually get randomly inspired to draw at odd times and sketch out something from my head. Then if I feel it's something worth following up on go back over it, fix proportions, look up accessories/items I drew and find how they actually look and model from there. Just be careful not to remove that 'stylized feel' your artwork may have by trying to make everything realistic (unless of course you're going for ultimate realism), I've found that just tends to take something beautiful and make it look like something's missing once it's in 3d. I had that problem for a few years =