It really depends on where in the Portuguese speaking world as it is even different from place to place within the individual countries (generally, more rolling closer to the Spanish speakers...):
Generally, in Western Portugal the double "r" has two sounds. An "r" followed by an "h" sound (sometimes gutteral). But neither are like English. Definitely a "tap" in there, but not a trill.
Here is a phrase at Forvo that has several different "r" sounds (but only a couple speakers, both from Brazil so not as representative).
All Brazil here too, but definitely a rolled "r" from one:
Completely different accent here (still from Brazil):
Total "h" sound even at the end:
Some more just for fun (first 3 all same person):
Someone from Portugal here:
Here is one finally with more than one from Portugal (even one from Spain) :D
This letter is difficult in any language it seems, unless it is your native one.
When it's about adjectives and nouns, it's about being literal or abstract/sentimental.
It's not always like this, and some adjectives might take only one position, but in general it works like this:
- Uma casa nova = A new house (recently built)
- Uma nova casa = A new house (somewhere I recently moved into)
- Um amigo velho = An old friend (a friend in advanced age)
- Um velho amigo = An old friend (we've been friends for a long time)