Why does the co-pilot sometimes keep a hand on the throttle as well as the pilot?
The pilots were probably executing a Cat II coupled or monitored approach. The PF is flying the plane (via autopilot, usually), while the PNF had his hands at the base of the throttle ready to take over to initiate a manual go around (or land in some cases, this varies from airline to airline).

There is a thread regarding this on this on airliners.net. It gives one version of the procedure:


  At my airline we use this procedure for Cat II approaches. The FO always flies the approach... well the autopilot flies it and the FO monitors the instruments, ready to perform a missed approach if the approach becomes unstable. The Captain at the 500 to minimums call places their hand near the base of the thrust levers, under the FOs arm, and focuses outside looking for the runway. If at any point prior to minimums the captain gets the runway in sight, they call "landing, my controls" and move their hand up to the thrust levers, bumping the FOs arm and hand up and off of the thrust levers. If at minimums and the runway is not in sight the FO performs the missed approach.