The standard Japanese dialect "R" is a voiced
alveolar tap/
flap. A flap is like a stop/
plosive(/p/, /t/, /k/), except the passage is closed for a much shorter time.
Flaps are often found in non-southern
American English as allophones of /t/ and /d/, such as in bi
tter, wa
ter, pota
to, la
tter, and la
dder.
In short, it is never an American English "R" sound.
Sometimes, such as when spoken slowly, or at the beginning of a word, or in songs, the Japanese "R" may be pronounced as a
lateral "L" sound by some people.
In the mostly extinct dialect of downtown
Edo, the "R" is sometimes
trilled. In anime, a character may switch to this form of pronunciation when shouting angrily, to depict a rude
Edokko stereotype. One thing to keep in mind when watching anime is that the Japanese spoken in it may not accurately depict
vernacular Japanese.