opto isolator and capacitors

Hi,

I'm putting together my breakout board and am using an opto isolators.

A friend said that I should use a small cap at the switch and a
resistor in the power line (5 volts).

I assumed that they worked just like a LED. figure current and
voltage then select a resistor.

Is he correct that a cap should be used too ? And if so, should it
be at the switch or at the board ?

Dave