Alter another field's input state via selection?

[Seabody]

Is it possible to alter another field's input state (no idea what to call it) depending on what's selected?

For example, assume I have a listbox and a textbox. The listbox is a required field, the textbox is not editable. The listbox has values A B and C. If C is selected, can we change the textbox to required?

I know it's possible to do it by overriding the listbox if the textbox is full, but that confuses the end user - if one is not editable then becomes editable (value C allows the user to input a custom version), it's easier to understand.

Sorry if I started rambling - if you're unsure WTF I'm on about, just tell me and I'll try to explain again.