Paul Rudd will star in the season finale of Moone Boy on Sky One next Monday.
The star of Anchorman, Knocked Up and the forthcoming Ant-Man was first announced to star alongside Chris O'Dowd last year.
In the episode, Martin and Sean discover that Grandad also has an imaginary friend.
https://twitter.com/BigBoyler/status/583314408522190848
Other cameos in this series have included Terry Wogan and comedian Sharon Horgan.
