Pete on "Only post author and commentor can view their own comments"

Only post author and commentor can view their own comments.

How would I do this...
I want the comment's to an author's posts ONLY viewable by the post author and the commentor.

For example the author publishes a post, commentor A posts a comment on the post and the post author post's a reply comment to commentor A. ONLY the post author and commentor A can view their comments. If commentor B does the same then only the post author and commentor B can view their comments.

Thanks
Pete