When the musicians at the Prancing Pony begin to accompany Frodo's song, Frodo's voice obviously switches from being recorded live in the studio to being prerecorded.
According to the English subtitles, Gandalf's letter refers to Aragorn as "skinny". This corresponds to the letter calling Strider "lean" in the book; however, it is not an accurate description of Kari Väänänen, who plays Strider.