Well when you start having a first read in a script , when you have characters such as the protagonist, and the secondary ones that get to be very important in the story. You need to get your visual development ideas moving, for this FIRST I recommend explore what are you going to do?
1- Go just for pencil ideas, (either traditional or digital) no render no color no buts!
2- You must first get the exploration sketches meaning sketching a personality, this comes right as your first impression, some artist need research before the drawing if they are not to familiar or over exposed to the idea..
3- Get some realistic approach first , this helps you imagine better what do you want to do with the style of the character later.
4- Just go over the age that the character shows at the story , this way you don't loose time having to trim down more ideas that make the process slower and sometimes not needed...