Question: everyone. 

When referring to a person, are there any firm guidelines when it comes to the use of "a" or "the" after the comma that follows their name? 

To clarify: 

Recently, I was writing an article about Amy Winehouse, and I referred to her like this: 

"Amy Winehouse, the troubled singer-songwriter who..."

A few weeks later, however, I was writing an article about a celebrated photographer, and it felt wrong to use "the" after the comma, so I wrote: 

"Sebastiao Salgado, a celebrated Brazilian photographer who..." 

As a result, I am now wondering if there are any solid guidelines? I realise that "a" is used when the subject is one of many, and that "the" is used when there is only one, but there have been times when I have followed these guidelines and the sentence has looked / felt wrong. 

Can anyone offer some guidance?

Answer: 
I think it comes down to how well-known you consider the person to be as far as the reader is concerned.

"...the famous singer..." assumes your reader knows the person. Additionally, it may distinguish them from another person of the same name.

"... a famous singer ..." assumes the reader is being introduced to the person.

**Example**

"By an amazing coincidence John Smith the famous singer had his house burgled by John Smith a petty criminal."
I like your rule and think you should trust it. 

I also offer you the thought that when you write "... , a troubled ...", you are writing as if the reader does not already know about the singer, whereas when you write "... , the troubled ...", you are writing as if the reader already knows about the singer.