How can a single Curve Interpolation Mode set to Constant? In order to animate a character so that it grabs an object an throws it, I added a Child-Of constrained and keyframed its influence. In the F-curve Editor I can't set the curve which controls the influence to interpolation-mode Constant without changing the other curves (Which should be linear or bezier). 

How can the interpolation mode of a curve changed independently?
You can select left-click-select every single Key in the Graph editor and press T to change the interpolation. This will always only affect the interpolation up to the next key of that one curve.

Further controls for this:


A deselects all keys, just like in edit-mode. 
L selects all keys of the curve of which one key is currently selected.
Tab locks the selected curve to prevent it from being edited.