The interrupt function (or handler) needs to do something very simple: at a specified (and hopefully user-controlled variable) interval, check a certain global variable and write to a servo following certain conditions.
What else is the Arduino doing?
How accurate (plus or minus how many milliseconds, microseconds, or nanoseconds) does the timing need to be?
What size interval, in milliseconds, microseconds, or nanoseconds?
If the answers are not much, not very, and fairly large, than interrupts are not even necessary.