The voice and motions were done by Andy Serkis, though Gollum's image was computer generated.
Gollum/Smeagol
Arwen's song was actually sung by Liv Tyler, the actress that played Arwen in the Lord of the Rings movies.
Jason Isaacs was not in the Lord of the Rings movies. You may be thinking of Hugo Weaving, who played Elrond.
lord of the rings
Billy Boyd played Pippin in The Lord Of The Rings.