The story I'm writing begins with the main character in an unusual and demanding situation. For various reasons, this situation causes him to behave in ways that are quite different to how we see him act for the rest of the story.
Now, if this were happening later in the story, I'd simply establish what has changed and why it's unusual for him, and provided I'd characterised him right, his unusual (for him) behaviour would seem natural and appropriate, given the change of circumstance.
My trouble is, since the character is going back to far more familiar conditions, and isn't self-aware enough to think about or acknowledge this fact (at least, not without prompting from another character), many of the usual tricks I would use to show that his relationship to his environment has changed will be unavailable to me.
How then do I flag up the relevant changes, such that the character's behaviour doesn't seem arbitrary to the reader, without "breaking character" (in the sense of revealing things from the character's perspective that the character himself would pay no attention to)?