I am able to pronounce the r in pretty much the standard way, but I still feel the faintest discomfort, especially when there are many r sounds in a row. On the other hand the trilled r is very natural for me. Lately I have been listening to several English productions, such as this one:
The trilled r can be heard several times before 1:45. If actors are permitted to do this for clarity, it should be acceptable for me as well. Yet, it seems, even the speakers who trill the r do not trill all occurrences. For instance, around 2 hours 51 minutes in the aforementioned video one of the actors always rolls the r in "prosperity", but not in most other words. Furthermore, please consider this video right after 3:45. The Dowager Countess rolls the r in "experience", but not in a lot of other words! Therefore my question arises:
When using the older standard pronunciation with the trilled r, which is apparently still used by actors, may we trill all occurrences of r, except such that are part of a final er (as "father", "mother"...)?