Most of my work starts off as a doodle. If I have something particular in mind it's more of a sketch but 99% of the time it's a biro doodle. Things tend to just grow on the page from an initial scribble of an eye or shape. Then I'll sketch it up a few more times in biro and then trace it with blue pencil, followed by a brush pen or a liner. It's surprising how many artists and illustrators use a biro or similar as their go-to sketching tool.
For me, this stemmed from picking up whatever drawing/writing implements I could find left around the house when I was a little tacker. I also drew on anything I could find. If there was no paper or sketchpad it was an envelope or a wall.
All those materials were good enough when things were tight but there was one combination that was pure bliss. I've never found a combination quite like it and I know others feel the same way. It seems like a dirty secret from an age past but as the great Halfman Halfbiscuit sang, 'There is nothing better in life than writing on the sole of your slipper with a biro' and there isn't...well, nothing I want to talk about on here anyway.