Yes, you can update a particular field using WorkFlow Assignment Values. Goto Setup > Workflow Settings > Create a workflow rule. Then create a workflow assignment value and associate that to the Workflow Rule. For detailed help to create workflow ...
No, currently it is not possible. You can make the field either visible or in-visible. To hide/show fields, Goto Setup >Admin Settings > Field level security >select profile & module to hide/show fields For More refer Field Level Security.