I have politely placed my hand next to the flap lever to kindly remind the old bloke flying that we are approaching GS capture and that if we don't get any drag out somewhat soon-ish, then we're NEVER gonna slow down! And, in situations when I've been under the pump (usually the sim), a colleague placing their hand next to the flap lever, has politely reminded me that a bit of drag might help the situation.
I wholeheartedly agree with Checkboard. Backseat driving by sneaking up on the flap lever is not on. If your oppo's forgotten something, just tell him/her.