The practice of dialect coaching includes and has developed its own terminology over the years. Some of the terms I use in articles on this website are defined below.
RP might be thought of as the standard British accent. RP is the most thoroughly intelligible and unremarkable form of spoken English. RP is the best communicator, used by politicians, voiceover artists and actors. For an actor, a really detailed knowledge of RP provides the base from which all accents and dialects deviate. In that sense it becomes the first and most useful accent apart from his or her own native sound. The best example of RP speech is that used by BBC news readers and announcers and recently the term "RP" has become interchangable with "BBC English." RP is so common as a title, it will be some time before it will be completely abandoned.
A good source for additional information about pronunciation is The Cambridge English Pronouncing Dictionary by Daniel Jones, published by Cambridge University Press.
In rhotic varieties of English pronunciation, the "r" consonant is pronounced wherever it occurs in spelling and, depending on the accent or dialect, in varying strengths.
In non-rhotic varieties of English pronunciation, the "r" consonant is pronounced only when it occurs before a vowel.
This can be thought of in different ways. Technically, it is the vocal sound produced by an individual influenced by the construction and use of the vocal apparatus. For example, "nasal" where the sound of the voice is resonated naturally or deliberately through the nasal cavities. "Rough" where the sound is produced by pushing down on and tightening the muscles of the larynx. "Deep" where the chest is used more to resonate the voice and the lower tones. Often we use words that imply colour or textural sensations, for instance "light", "dark", "bright", "soft", "hard", "velvety", and "rich." For an actor, vocal quality will often inform a dialect, and vice versa. Frequently they go together in the creation of a character.