How do I make a field read-only for some users but editable for other users?

There is a standard field which we would like to have displayed for all users, but the field should only be editable by some users. How can I achieve this? We are on R12 and these fields are not in Forms, but in the web-interface.

Do I need to create different responsibilities for this or is there some way of setting field level access directly on users?

As you probably can make out from my question, I am not that familiar with how the security settings works in e-Business Suite. I have searched through google and some user guides but I have not found anything on this unfortunately.