Jason Isaacs is a British actor of some note, as well as being a classmate of Mark Kermode. Having once admitted to being an avid listener of the show, he was granted the honour of receiving a "Hello" from the presenters on an almostly weekly basis.
Since Jason always replies back when he hear the greetings in the podcasts, the good doctors often leave a gap of silence after their "hello". Jason has told stories of being in weird/awkward public places when replying and even being in a full Death Eater costume at the Harry Potter set.
When Jason starred in the American TV series Awake, he snuck in a greeting for the listeners of Wittertainment in the form of the word "ukulele" picked by one of the listeners.
Saying hello to Jason has subsequently become a bit of a meme, spreading to fans greeting him with cries of "Hello to Jason Isaacs" on the street, and signs declaring the same appearing at sporting events, protests, etc.
A Google engineer who listens to the show has made the UK desktop version of the search engine respond to the query "Jason Isaacs" with "Hello to Jason Isaacs".