Yep. That's why it helps to keep any scripts as well as plotline flexible so to allow for any new ideas that pop up.
Some characters I never expected to use much suddenly proved themselves to be useful. Roxanne, Izabel, Carrie and Sheila are good examples.
On the other hand, some characters I intended to use aplenty, didn't work out so they "left" early (Christopher, Jacobs, Travis)