play a part in=play a role in 指在.中起作用.part 前常加形容词来进行修饰,例leading,important,critical,decicive等.例句：with the development of science and technology,computers have played an important part in our daily life.
而play the part in 之在.中扮演.角色完整用法为：play the part of...in ...例：
Oliver played the part of Hamlet in Shakespeare's drama.