Please clarify the following:

I could not find any difference (in privileges) available for Reporter
and Assignee in bugzilla 3.0.2.

Reporter can register a bug and if he wants to reopen the bug/verify
the bug/close the bug, he can do.

Similarly, Assignee also can edit all the bug fields.

Is there any way to restrict Reporter/assignee to certain fields only?

i.e. Assignee should not change Priority and severity fields.