I've seen three movies Ian has played in Lord of the Rings: The Fellowship of the Ring, The Return of the King, and The Day After Tomorrow.
Lord Of The Rings - He played Bilbo Baggins, he only played in the first and last ones and he didn't have that big of a part but his performance was superb, he made Bilbo's suffering because of the One Ring, believable and he made you hate and love his character at the same time truly remarkable.
Day After Tomorrow - played the part of Professor. Terry Rapson, and it was another small part, but this character was a bit more serious manly do to the nature of the film, and the fate of his character is sad. No matter what type of character Ian plays he always gives a superb performance.