Then, he went to London to become a comedian and now he is one of the most famous stars in that field.

comedian 是由名词 comedy（喜剧）加上后缀 -ian 构成的名词，表示"从事喜剧表演的人"，即"喜剧演员"。-ian 这个后缀常用来表示"从事某种职业的人"。