Doctor, "I'll explain later."
Companion, "No, you won't."
D, "Yes, I will!"
C, "No, you won't. You never do. You always say that, but you never do."
D, fidgeting with something, "I will."
D, "Later. Just not now."
C, "This is the problem with time travel! Everything is later!"
C, "Why can't it happen now?"
D, "You can't have everything happen in the now. It would all crash together and whoosh!"
D, "Ah. You see? When I do explain something you always give me that look. I don't know why I ever try."
Mostly some menacing alien would step in at this point or the mechanical thing he was fidgeting with would start working and distract the companion from head-exploding or Doctor-throttling.