Harking back to the comments on observation of an aircraft's flightpath and speed, if I ever had a pilot on the jump seat, I would always give them a headset and ask them to be my 3rd pair of eyes and ears. On at least 2 occasions this 3rd, supplementary pilot helped to sort out a couple of unclear ATC transmissions.
Also, if I flew with a new F/O, straight out of Line Training, once I'd completed my departure brief I would say:
"There's one important thing I need to add which is this. The reason I'm in this left-hand seat is because I've been doing this job longer than you. It doesn't mean that I'm incapable of making mistakes. So if you see or hear anything which you don't understand or appears to be not right, please speak up and tell me."
My company had excellent CRM training, but I still felt that it was important to give a new and maybe nervous young F/O full permission and encouragement to say something if they weren't comfortable. There is absolutely no room for pride in the flight deck; the important thing is that the day's flying is accomplished safely, and hopefully enjoyably as well.
And back to the posts on unserviceable ILS at LHR, I do remember a day, maybe 15 years ago, when the ILS for 09L was out and pilots were invited to make NDB approaches. What ATC had not appreciated was that company policy of some airlines was for the approach to be flown at approach speed from the final descent point. Some 10 miles at maybe a Vref of 125 knots rather than 160 knots, sure slowed the landing rate down! I seem to remember that the 4 holds were well used that day!