Actor Ajay Devgan did just that recently.
The news is that there was a cameo role in the film 'Teen Patti', being produced by Ambika Hinduja and directed by Leena Yadav. The makers initially wanted Akshaye Khanna, but he had to turn the film down. So they started looking for another actor.
The word is that Akshaye was called at the last minute to dub for 'Shortkut', where he plays the lead. The actor had no other option but to go ahead with his dubbing, leaving the makers of 'Teen Patti' in trouble.
But the problem was not many actors dared to act in a film which is dominated by the Big B. So many remained uninterested! That's when Amitabh came to their rescue and use his personal charm.
Not surprisingly he succeeded as actor Ajay Devgan instantly agreed to do the cameo and shoot for a couple of days.