In speaking styles, there is a continuum between mumbling and rolling your ‘r’s –. What I mean by mumbling here is speaking with as little mouth movement as possible in order to minimize effort on the part of the speaker. ‘Rolling your r’s’ on the other hand is my shorthand way of referring to hyper-articulating - making as much effort as possible to pronounce every phoneme in the word, in order to minimize the effort that is required of the listener. There is a tension between these two extremes. That which reduces the effort for one participant increases it for the other. The participants in any interaction need to find the right balance. This sweet spot is usually going to be somewhere in the middle, a compromise, so there will be a central bulge in the continuum, with most interactions in that area. So how do speakers decide how carefully to articulate? You might think that the safest bet would be to always articulate to the maximum. However this is not the case. If you hyper-articulate in a situation where that is not expected, people are likely to think you’re being sarcastic or expressing some double-meaning.
What we need is a guide, which will tell us where to pitch our speech style. It might begin something like this...
Am I with very close friends? = I can mumble a bit.
Am I at a job interview? = Speak carefully
Am I a Shakespearian actor? = I’ll roll my r’s.
Am I a teenager? = Sloppy’s good.
Am I making a formal speech? = Hyper-articulate
Am I an English teacher? = ...(you get the picture!)