The entire point of ristretto is to make espresso with little or no bitterness. Usually used only for cupping, to get a better idea of the bean's flavor. Although it's also some coffee junkie's ideal way of drinking espresso.
In general, a ristretto has less volume than normal espresso. Like a short pull, but not exactly. To make a ristretto the barista grinds the beans extra fine, to the point that the equivalent tamp and dose results in a 30 to 35 second pour. Using either a single or double filter basket filled roughly the same as for a normal pull, one can make either a single or double ristretto, though the volume will differ.
A lighter crema indicates more bitterness, a good barista will recognize just when the grounds are becoming overextracted based on the color, and cut off the pour.
Sadly, most minimum wage, part time Baristas in North America, (as if we have any other kind) have no clue what a ristretto is, let alone how to make one. The closest I've come was one guy who thought I meant a half shot, prepared normally. He could not comprehend the idea of a double ristretto.
Please note that if you try this at home, much experimentation, as well as caution, will be required. Many grinders have a hard enough time grinding fine enough for regular espresso, let alone ristretto shots. If you have a cheap espresso maker, grinding extra fine runs the risk of "choking" the machine. And unless your espresso machine is equipped with a 3-way solenoid valve (to relieve pressure), you could end up with either a broken pump, or a portafilter sneeze. I've had my kitchen repainted with coffee grounds more than once. Luckily, I wasn't in the room.