But on set it is Stephen Mangan, as the comic-grotesque Dr Guy Secretan, who emerges as the true star. With film roles in Festival and the forthcoming Confetti, it can't be long before he's up there with Coogan and Gervais. The rest of the cast are all straight actors, but Mangan is a brilliant improv performer who pushes the script to the very limit with his constant ad-libbing.