Change Letter (sometimes referred to as Letter Changes) was a game played on both the British and the American editions of Whose Line
The game was a straight improvisation of a scene, with the constraint that the performers had to replace a particular letter with another one - usually an unrelated letter - such as replacing the letter B with the letter F. Sometimes, each performer had a different letter to change, but most of the time, everyone changed the same letter.
On the British version, two improvisers acted out a basic scene. On the American version, all four improvisors had to act out a much more specific setting.
The game was played very differently by different performers. Brad Sherwood and Ryan Stiles revelled in the rule, often trying to say as many words with the changed letter as possible (the discussion about "fasketfall" and the star "fasketfaller" called "Fof" being a prime example). Colin Mochrie, on the other hand, spent most of his time trying to avoid the letter where possible, speaking very deliberately - although he was involved in the "gate" exchange when P was replaced with G (going as far as wondering "Am I sgeaking a different language?"). Brad also spent a lot of time trying to get the other performers to swear, such as by asking Wayne Brady what he'd been trying to shoot (a male deer) in the emergency room playing.