"regular" interrupts will see the event happen be it an internal timer or change on a pin. Then it waits until the program is finished doing what ever then goes to the interrupt routine. This is the timing issue spoke of. That is where the clock programs come in for an explanation, there is only so much time between interrupt events.

If the program is in the middle of a serial command and an interrupt flag is set, the interrupt will not run until the serial routine is finished.

Here is a recent thread where an interrupt is being worked out.
http://www.picbasic.co.uk/forum/showthread.php?t=7490