As was said its a two way deal. The fish has to be willing to be cleaned. This would involve the fish going to the 'cleaning station' and allowing the shrimp to hop on and start working its magic.
The shrimp really wong force itself upon the fish nor will it really go activly searhing for fish to clean. More or less they just setup cleaning stations where they will service a fish if it comes up and allows the shrimp to 'pop the hood' so to speak.