Depends on what you mean by "visible but not editable". If you mean "disabled when field #2 is a certain value" then you can't do this using standard Salesforce. (This is easy using Visualforce, but of course you have to develop a new VF page, which may not be so easy.)
If you mean "when field #2 is a certain value, edits to this field cause an error message to be shown" then you can easily do this with a validation rule.
too generic. but if it is standard page you need a validation rule on the second field.
Depends on what you mean by "visible but not editable". If you mean "disabled when field #2 is a certain value" then you can't do this using standard Salesforce. (This is easy using Visualforce, but of course you have to develop a new VF page, which may not be so easy.)
If you mean "when field #2 is a certain value, edits to this field cause an error message to be shown" then you can easily do this with a validation rule.
Good luck!