In the previous step, you added characters implied by your synopsis to your potential cast list. Range a little wider now, and jot down some characters that aren't explicitly mentioned or even implied but wouldn't seem particularly out of place in such a story.
In the example story we’ve been using, no one would be surprised at all to encounter a saloon girl, a bartender, blacksmith, rancher, preacher, schoolteacher, etc.
There is no specific limit to how many or how few “usual characters” you can or should add to your growing cast list. So just add the ones that appeal to you.
Don’t be worried if any of your additions seem stereotypical of too predictable. By the time we’re through a few more steps your list will be so large we’ll need to pare it down.
So for now, beef up your cast with any additional characters that would fit right in your novel as described in your synopsis.