Prior to 9/11 I was priveleged to be in the cockpit going into Heathrow. A combination of circumstances (an emergency going to 27L when we were on 27R, something falling off a departure from 27L maning the aircraft that had lined up on 27L could not depart so the emergency ended up heading for 27R) , on approach, beyond the control of the crew meant we ended up much closer than normal to the preceeding (emergency) aircraft. As the situation unfolded our captain reduced speed as much as he could and announced that over the radio. The aircraft behind us replied "I can't fly that slow".
We got clearance to land at the very last moment (we were crossing the permiter road), I had watched the captains thumb (?, it might have been another finger) moving towards the TOGA button and was expecting a go-around. Instead we landed with full reverse thrust, maximum autobrake etc and exited the runway in time for the following aircraft to land. I had colleagues in the back of the aircraft and when I asked them how the landing was they were amazed by what had happened, they had no idea.
Unless you are in the tower or, more likely, cockpit it's almost impossible to know what was going on.
Here's another example. Landed at O'Hare in a 747. Very heavy braking and quick runway exit. The captain came on as we taxied towards terminal 5, apologised for the heavy braking and then went on "the aircraft in front was a Chinese 747 and I thought you might like to get into immigration before they do".