What happens to a diesel generator as you apply load?

When you apply load to a diesel generator, the generators component parts react to the change from steady state proportionally to the amount of load applied. The AC alternator will react to the application of load by the voltage dipping at the terminals simultaneously. The diesel engines speed will reduce when the load is applied.

While the Diesel Generator is running at no load the engine is able to spin the alternator quite freely, as there is little resistance provided in the magnetic fields within the alternator. 

For increases in load that are greater than trivial in size you can hear the engine sound change as the speed initially falls and then recovers, assuming the engine is not now overloaded. At steady state it is the job of the engine governor to control the speed of the engine, preventing the speed from increasing uncontrollably, or dropping too far below its rated speed. As load is applied or removed the governors job is to return the engine to its rated speed. It does this by adjusting the actuator controlling the fuel supply to the engine.

At level of load greater than zero, the engine will be louder, with the sound level peaking at its maximum load. This additional sound energy comes from the larger more powerful explosions of diesel and air in the engines cylinders,  turning the chemical energy in the fuel into additional mechanical energy to rotate the alternator as the magnetic field between the main rotor and main stator becomes stronger and its resistance to turning increases. The engine will consume more fuel, require more air and produce more heat from the combustion process.

The AC alternator voltage dip in comparison will generally only be detectable using a multi-meter or by monitoring the generators control panel. If the voltage dip is large, because the applied load is large, then lights may flicker and sensitive electronic equipment may turn off or become damaged by the voltage drop. The alternators AVR and excitation system will react to increase the flow of current to the main rotor, generating an increase in voltage in the main stator and at the generator terminals.

Due to the engine and the alternator being direct coupled, the change in the engines speed will directly effect the frequency of AC current the alternator is producing.