What expression pedal are you hoping to emulate, or what effect parameter are you hoping to control? Most exp pedals are a potentiometer with the wiper wired to tip, hot (lug3) to ring and cold (Lug1) to shield. They are usually 10k-100kB. A single photoresistor/LDR can often act as an expression device if you only need to vary a single resistance, but you will need some active circuitry to either invert the response (so lowering your foot over the LDR replicates the action of lowering your toe over a rocker pedal) or to have a single LDR accurately replicate the action of a voltage divider (i.e. two resistances that shift in opposite directions and three connections, of which one is common).
Andrew "thetonegod" has written a good article on exactly this subject:
http://tech.thetonegod.com/rockncontrol/rock.html