There are varied opinions on the amount of research that should be undertaken. For the typically high volume of arias to be performed in a concert (perhaps 9 or 10), it is unrealistic to recommend that every opera should be studied in detail.
However, I assess that a few key elements should be known which will inform performance:
a) Musical period of the opera’s composition
This gives the singer an indication as to the appropriateness of cadenzas, ornaments and use of portamento, depending on the period. For example, it will be expected that in a Baroque da capo aria, that the singer use decorations on the repeated first section.
b) An overview of the composer’s life and when the opera was composed in his/her œuvre (first, last, unfinished etc)
Understanding this will help to decode some of the choices made. For example, A Midsummer Night’s Dream was one of Benjamin Britten’s later operas written in 1960. It is also known that although he often wrote main roles for Peter Pears, in this opera, Pears had a relatively small part. The main male character, Oberon, was written for popular counter tenor of the day, Alfred Deller, who is well known for having brought the counter tenor voice back into fashion. This information helps the singer to know the wider context and therefore informs their own approach. Alfred Deller could sing long legato lines, so many of Oberon’s solos have been written with that in mind, for example, his aria I know a bank.