NEW YORK – Ewan McGregor was having a hard time keeping up with Christopher Plummer. At the suggestion of director Mike Mills, McGregor had brought his 81-year-old co-star to Barney's in Los Angeles to pick up a scarf for Plummer's elderly gay character in their film "Beginners." But Plummer had other designs. "Christopher only wanted to get skinny black jeans," recalls McGregor. "That was his main goal in life. When we got there, he asked where the jeans department was, and off he went to…