Nouns in apposition and nouns forming part of an apposition are used with the indefinite article if the speaker states that the object expressed by the noun in apposition belongs to a certain class.

I want to introduce you to Fuad., a great friend of mine.

In the plural, no article is used.

I want to introduce you to Comrades B, and D., great friends of mine.

Nouns in apposition or nouns forming part of apposition are used with the definite article if they are modified by a particularizing attribute.

Fuad, the student you have mentioned, has come.

If the noun denotes a well-known person or work of art, the definite article is generally used.

Pushkin, the great Russian poet, died in 1837.

Hamlet, the immortal tragedy by Shakespeare, was written in the first years of the 17th century.

But if the person or the work of art is not widely known the indefinite article is used.

Pericles, a comedy by Shakespeare, is hardly ever staged.

No article is generally used with a noun in apposition when the opposition expresses a post that can be occupied by one person at a time. Occasionally the definite article is used.

Professor Petrov, director (the director) of the Medical Institute, is going to deliver a lecture.

Comrade Smirnov, dean (the dean) of the English department, has left for Moscow.