About combining behaviors with keyframes
Any object can have behaviors and keyframes applied to it simultaneously. When this happens, the values generated by the behavior and the keyframed values applied to the parameter are combined to yield the final value for that parameter. This lets you combine the automatic convenience of behaviors with the direct control of keyframing to achieve your final result.
Note: Motion has a specific order of operations for keyframes and behaviors. For more information, see About behavior order of operations.
For example, if you create an animation path using keyframes, you can create a completely predictable and smooth movement.
However, if you apply the Randomize parameter behavior to the same object, its effect combines with the keyframed motion path you created. As a result, the animation path follows the general direction you want, with random variation in it to make it interesting.
Although this example shows how you can combine behaviors and keyframes to create animation paths, you can combine behaviors and keyframes for any parameter.