Speaking of Spikes part in 'Life of Brian'

He was in Tunisia visiting WWII sites and heard the Pythons were filming not far from where he was, so just went along for a visit.

Too good an opportunity to miss, they quickly wrote a cameo part for their all time hero.

He plays a Prophet, who's believers all leave him to go and follow Brian, much to Spikes characters displeasure who then warns Brian "It will all go wrong, you'll see".  If you listen carefully that bit is meant to lead to another scene, which Spike was meant to film the Day before He left to return to the UK after visiting the War sites.

Brian is on the cross and you see a figure slowly approaching from the far distance (something often used by the Pythons in the TV show) but this was meant to be Spike, who then says  "I did warn You"

But Spike being Spike totally forgot and went home. When they finally got a hold of Him, He had already started doing something else and couldn't come.