As I understand it, Johnny has no obligation to confirm anything exculpatory that you say during questioning. He can and will use anything incriminating, however.
No obligation to
volunteer the statement, no. But if asked a question that would call for that information, and he withholds it, he is perjuring himself and is in contempt of court.
Not really true. There are plenty of different hearsay exceptions that could apply. Plus, the defendant can take the stand and just say what he or she wants to say.
But that's the point: I cannot, at least according to my reading of Federal Rule of Evidence 801 (d), use prior statements of mine for my defense unless I have given testimony related to it. By default, the statements would be inadmissible (according to how I read the rule and according to what I'd seen in a presentation by a law professor).
Again, that is not exactly true. FRE 801 only covers things that are
exclusions from the hearsay rule--i.e., things that are considered
not hearsay. There are other rules that are
exceptions to the hearsay rule that would let statements come in even though they are hearsay. In other words, they are hearsay, so the general rules is that hearsay does not come into evidence, but even though they are hearsay, there are exceptions that let them come into evidence anyway. As those rules apply, there are TONS of situations where a witness can testify as to his own prior statements. And, again, even aside from that, even in situations where you cannot testify as to the statement itself, you can still testify as to the facts. So let's assume you couldn't testify as follows (which isn't true, but let's assume): "
Then I said to him, 'Oh, it looks like you have a gun!'" You would still be allowed to testify: "It looked to me like he had a gun."