Some years ago, Alan Weeks and I got obsessed with frequencies of names. Alan did the analysis and found that the most common names in North Baddesley were Patricia and Margaret. Later, I took this further by predicting demographics on the basis of name frequency: there is a fashion in the naming of children, as in so much else, so you can make a reasonable prediction of age based on popularity of name. (Margaret was most probably a child of the 1930s or 40s, Patricia peaked in the 40s or 50s.)
This becomes relevant to play-writing in the business of naming characters. Names need to match the ages of the characters in the era in which they are set. So, for example, in England, peak Susan happened in about 1955. That means that for a play set in 2014, you would expect most Susans to be in their late fifties to early sixties. A Susan who is eighteen seems out of place. Now, of course, if you want to disturb your audience, then giving a character an unexpected name may be a legitimate ploy, but if you want immediate acceptance, then the name, age and era need to match.
Being fashion accessories, names are cyclic. Violet went out of fashion in the 1920s along with Doris but seems to be staging a come-back. Sam (for men) peaked even earlier, alongside Phyllis (not for men!) and started a major return in the 1980s. Being cautious about this, there are always a few outliers. There are the out-of-era names given in honour of an ancestor, and there are the names that don’t really fit in any era: I was idling after a tour of Monterey Bay Aquarium when I overheard a mother admonish her child with “Stop that, Amadeus!”
Fashions for names vary enormously with geography. I was careful to specify England with respect to Susans. I once called on someone named Phyllis, expecting to be greeted by a nonagenarian only to find a Scotswoman in her early thirties. Names in the USA are more diverse and are not synchronised with England; hence I was once discomforted to find myself working with a Doris who was younger than me. It becomes even more difficult when dealing with names from non-Anglophone countries. There was, for example, my business correspondence with a Dutchwoman called Anne; turned-out to be a bloke with a moustache.