‍▲曾老师的分析很好！但这个问题，我们也可以从另一个角度来理解，即将role of 后面的名词理解为一种“独一无二的身份或职位等”，其前的冠词是被省略掉了。比如英国著名语法学家夸克等人在《英语语法大全》中就有类似观点，夸克等人不仅认为 the role of 后面的名词通常省略，而且 the post of, the position of 后面的名词前也通常省略冠词。

▲关于这个问题，《英语语法大全》的原文是这样说的：

Because a unique role is implied, the definite article is generallyomitted following expressions such as the post of, the position of, and therole of.

该句大意为：由于隐含有“独一无二的身份”的意思，the post of,the position of, and the role of 这类表达后面的定冠词通常是被省略的。

▲类似例句：

She was keen to take on the role of producer. 她很想当制片人。

At our meeting, I took on the role of secretary. 我在会上当秘书。

Sally stepped into the role of team leader. 萨莉开始担任队长。

She was offered the position of sales manager. 她获得销售经理的职位。

Nominations are invited for the post of party chairman. 为党主席职位征集候选人。

I am pleased to tell you that your application for the post ofAssistant Editor has been successful. 我很高兴地通知你，你已经成功申请到了助理编辑一职。

His career culminated with the post of ambassador to NATO. 他的事业以驻北约大使一职达到顶点。

▲《英语语法大全》只是认为 the post of, the positionof, and the role of 这类表达后面的定冠词通常被省略，而不是说一定要省略，所以有时也用冠词也是可能的。如：

I have been relegated to the role of a mere assistant. 已经把我降到只任助手的工作了。（牛津词典）‍