What is the difference in meaning between the following two sentences?
(1) I am completely honest with you right now.
(2) I am being completely honest with you right now.
Verbs like be and know and see, which express a state are far less likely to be cast in the progressive than verbs which express an action or event. The primary effect of the progressive construction is to "recategorize" an event or action as a continuing state, so its use with stative verbs is superfluous.
When stative verbs are cast in the progressive it usually signals one of two things: either the "state" is regarded as temporary and likely to end soon, or the verb has been recategorized semantically: it has a different sense in the progressive.
In the case of be, both these changes take place with the progressive: be is understood in the sense behave, and the behavior expressed is understood to be temporary rather than a permanent characteristic. For instance,
Brian is a jerk means that Brian is always a jerk, but
Brian is being a jerk means that right now Brian is behaving like a jerk.
So it is one thing to say I am completely honest with you—this is the ordinary stative use of be and implies that I am always completely honest with you. It is a permanent aspect of my character. But I am completely honest with you right now is odd and jarring, because bare am and right now imply two different time scopes.
But saying I am being completely honest with you right now is idiomatic. This says that even if I may not always be completely honest with you or with other people, what I am saying and doing right now—my current behavior—is in fact completely honest.
† And in fact the only reason I would say this would be if you gave some indication that you believed I was not being honest with you.