When r comes before d, t, s, n and l, it's pulled back (towards the throat) and becomes the retroflex approximant "ɻ". Its sound is very close to the "rah, rah" part of Lady Gaga's "Bad Romance" song. In all other cases, r in Norwegian is single rolled (alveolar flap ɾ).
This change happens regardless whether it is touching the aforementioned letters or not. To make the pronunciation of the following letter easier, r also pulls the "d" backwards so it becomes the voiced retroflex stop "ɖ" (otherwise, it sounds as the voiced alveolar stop "d").
Some examples where this phonetic change happens are:
Spiser dere...? --
For a more accurate representation, click here and then on ɻ and ɖ.
Follow this link to watch a YouTube video where you're given explanations about five verbs (å snakke, å prate, å si, å telle, å fortelle) that all revolve about talking but are used in different situations and contexts. The video is in Norwegian so you might postpone watching it until you're at a more advanced level.