Golden advice.

I think there are moments before the climactic decision at which the audience can be kept in the dark about a character's motive. But like Chekhov's gun in Act 1, that question has to be answered at some point--either with an explanation for the choice, or with an explanation that the character acts capriciously or irrationally--without pattern or motive (and this latter "explanation" is not likely to endear itself to most audiences).

The latest explanation of behavior that I know of occurs at the very end of The Rover. The story is told so skillfully that even though the revelation is a bit of a gimmick, the audience doesn't feel cheated by the lateness (actually some people I know did, but I thought it was effective).