play the part of 也是个用法,意思是扮演了一个xxxx的角色.如扮演一个好人就说,play the part of a good guy.
而play a part in 意思是在xxxx（事件、范围内）扮演了一个角色,如在电影中扮演一个角色就说,play a part in a movie.
或者上面两个用法其实不互相冲突,可以用在一起,play a part of a good guy in a movie或play the part of a good guy in the movie.
play the part of a good guy中,the改成a也是可以的,按照定冠词和不定冠词的用法来决定用哪个就行了；但是play a part in只能用不定冠词a.