Hi,
an interrupt whenever the voltage on my analog input pin changes
The ADC takes a finite time to complete its conversion. When the conversion is complete the AD_INT flag is set. This means that you should always start your conversion manually. However your PIC16F88 has a compare module. That can automatically setup periodic conversion of your AD module. So you would be getting interrupts from the AD_INT that a new value is available. Just read the AD results and compare it with the previous one to find out if your pot has been changed. Please note do not expect two consecutive conversions to be exactly equal even if you are not touching the pot. Supply, layout, noise, blah, blah...... . So compare it for a change of value within a range.