On the face of it, due to the prestige, it should be easy for the RSC to get the best actors - and actors are always grossly over-supplied. And there are plenty of really inspired actors born - since most amateur shows I have seen have two or three outstanding, naturally-gifted actors.
(My brother Fraser, for example, is one - he can be seen on YouTube performing in Gilbert and Sullivan.)
But then the RSC acting is no worse than the general run of professional stage acting, which is poor - so the problem is systemic.
The phenomenon can be explained if you consider the levels of selection which operate on RSC actors before they get to the position of being seen by me. Here are some of them.
1. Whatever their talent, Shakespearian actors are made to act speak their lines badly by the directors.
At one level, a director is necessary nowadays to explain to actors what their lines mean. At another level 19 out of 20 directors make the actors speak blank verse as if it was prose, and thereby most of its effect is lost. There were reasons why Shakespeare wrote most of his drama in verse - and these reasons are ignored by speaking the lines as if they were not verse.
(But then, most actors can't read poetry even when performing it as poetry - so perhaps they must share the blame with directors; unless this wilful incompetence is a lasting consequence of their professional training.)
2. Acting is such a terrible job and lifestyle that most naturally gifted actors wuld find it intolerable to be a professional. This means that most of the potentially best actors are selected-out before drama school.
(My brother is an example - he became a medical graduate; then a histopathologist.)
3. The audiences are not good at specifically detecting when acting is good - because they are focused on the quality of the actor's voice.
Most professional actors cannot act - but nearly all do have pleasant, powerful, clear voices - because without that, then they are unsuitable for stage work. This requirement puts-up a significant barrier since the best actors are not likely also to have the best voices.
(I learned this from a friend who was a musical director for the RSC. I went to see his production on tour. The cast was headed by two really outstanding actors - Nigel Terry and Fiona Shaw - but almost all of the others fitted into the category of Great Voice/ Can't Act.)
4. The audiences focus not on acting - but on concepts, stage design, costumes and 'business' (the acting slang term for the non-spoken aspects of plays, esepcially comedy). In the RSC production I recently saw, by far the most memorable aspects were some very funny and surprising bits of business. One RSC trick is to include pretend mistakes, and pretend 'ad libs' - seemingly spontaneous but done every performance - the audiences appreciate the idea of actors having to use their wits get out of trouble.
5. The craft. Professional actors need a phenomenal memory to learn their parts and moves and to avoid mistakes - this is a major constaint because being a gifted actor does not necessarily go along with these vital craft aspects. They often need to be able to dance and sing - in order to make a living over many years - which some of the best actors cannot do.
Also, actors need to be able to turn-up and perform reliably, at an acceptable standard, night after night for months on end. Much of the uninspired natrure of professional stage acting comes from this aspect of routine - it really is impossible for actors to turn-it-on under such conditions.
Also, actors need to have certain looks - either being reasonably good-looking, or else interesting/ ugly in the right kind of way.
So there are plenty of reasons why the average of actors we see on stage at the RSC are merely adequate. But, when you do get a cast containing many really good actors (and assuming the production is not actively ruined by one of the numerous breed of narcissistic director) then the results can be simply wonderful!
I went to see the RSC Loves Labours Lost in 1978. This is not one of Shakespeare's best plays, by common consensus; but - as well as being beautifully designed and directed, the cast assembled was simply stunning; with a remarkable concentration of the very best actors of recent years, some already famous, others yet to make their mark*.
The result was a completely wonderful play, sheer magic and delight from start to finish. Despite that I have never again seen its like; this production was - of course - the real play. Because Shakespeare's own production company comprised one of the most gifted set of individuals that have ever been assembled on stage; and they performed to the most discriminating and experienced theatre audiences of all time.
(See Shakespeare on Toast, by Ben Crystal).
In conclusion, Shakespeare is nearly-always performed severely sub-optimally; and he is even better than we think he is!
*Including Michael Pennington, Alan Rickman, Michael Hordern, Richard Griffiths, Ian Charleson, Ruby Wax, Carmen Du Sautoy, Jane Lapotaire, David Suchet... has there ever been a better cast?