Roasting is one key to the transformation of the tasteless, raw seeds of an obscure tree from the Horn of Africa into the rich, resonant beverage we know as coffee. The flavor nuances imparted to coffee by roasting are particularly important in espresso cuisine, because the dark styles of roast used in espresso tend to mute taste characteristics inherent in the bean itself and replace them with characteristics generated by the roast.
The espresso brewing method is so efficient at extracting flavor from coffee that it tends to exaggerate flavor characteristics. Qualities that may be exhilarating or agreeable in other brewing methods can turn into irritating distractions when subject to the amplification of espresso brewing. The espresso method rewards deep, sweet, subtle flavor characteristics rather than those that are extreme or idiosyncratic.
Consequently, the best roasts for espresso brewing are those that are a bit darker than the American norm, but not too dark. Coffees roasted to the light through medium end of the spectrum typically are too acidy and bright, and tend to taste sharp or sour as espresso. On the other hand, extremely dark, almost black roasts come across as thin, watery, and charred.