Both are British (one English, the other Welsh). Both went to Oxford. Both have parents who decided to name them Rowan. And both have achieved worldwide fame. But in pretty much every other way that matters, they couldn't be more different from each other.
As Archbishop of Canterbury, Rowan Williams is the primus inter pares at the head of a worldwide religious community, the Anglican Communion. But he is also senior primate of the Church of England, an institution whose other leading figure, carrying the title Supreme Governor of the Church of England, is Queen Elizabeth II (one of whose other subsidiary titles, dating back to Henry VIII, is of course "Defender of the Faith"). By virtue of this role in what is not only a spiritual but a civic institution, Williams is a cornerstone of the British establishment. Which is to say that, in addition to being a shepherd responsible to fellow Anglicans around the globe, he is also a holder of a public office and of a public trust, responsible to the British Crown and to every citizen of the United Kingdom.
If Rowan Williams is that most solemn of things, the Queen's bishop, Rowan Atkinson is somewhere at the opposite end of the seriousness spectrum. Famous for his characters Blackadder and Mr. Bean, Atkinson is not just a comic actor but one who almost invariably plays the fool. Often, interestingly, these fools are clerics, such as the nervous vicar who, officiating at a marriage ceremony in Four Weddings and a Funeral, inadvertently says "Holy Goat" instead of "Holy Ghost."