In a few languages of Europe (French,German,Italian),these phonemes are in free variation. To my ears they sound quite distinct,but maybe it is because I lack sufficient knowledge about their acoustic properties and their auditory perception.I think the only thing they have in common is the "trill"of course,but I'd like to have a more detailed explanation. I apologize if the question is too vague.
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[r] and [R] are realisations in different German dialects. Any individual speaker will prefer one or the other to realise "r". That must be what "free variation" is supposed to mean. Or they are allophones in certain dialects. Anyhiw, often it will be rather an approximant, fricative or a tap, etc. The trills are very easy to differentiate, if stressed. There might be some overlap between them, for special realizations, which I would naturally not notice.
– vectory Apr 12 '19 at 19:43