From the Designer's Commentary:
Q: If any of your units are eligible to fight in the Fight phase, can you choose for them not to fight this turn? Also, if any of your units charged in the Charge phase, do they have to fight first in the Fight phase, or can you choose for them to wait until later in the phase?
A: All eligible units must fight in the Fight phase; they cannot ‘pass’ and wait for another phase. Additionally, a unit must fight when it is its time to do so; it cannot ‘hold’ in order to fight later in the phase. So if a unit charged in the preceding Charge phase, it must fight before any non-charging models in the Fight phase (barring any related abilities).
Note that when a model fights, it must do all of its close combat attacks if it can do so – you cannot choose for it not to do so (though you can still choose which weapon it uses for each close combat attack).