Just to keep things in balance, if more than one pilot is to act in a piloting capacity during a flight, that should be briefed before the flight begins. If such a briefing has not been conducted by the PIC, it is reasonable to think that pilot is not expecting to share the duties of the flight, nor the logging of time.
So thinking to "jump in" for some impromptu instruction should seem to any pilot as unusual, and perhaps unwelcome.
If a mentoring pilot wants to let you fly a while, how nice, accept the offer. But focus on the value of that as some familiarization for you rather than padding your logbook with a fraction of a flight.