Angelina stars as Mariane Pearl - the wife of murdered US journalist Daniel Pearl - in 'A Mighty Heart', and Brad, who is a producer on the movie, is helping her immerse herself in the role by attempting to emulate Daniel in almost every way, including learning to play the instrument he was accomplished in.
A source on set told Britain's Daily Express newspaper: "It's almost as though he's taken on the part of Daniel to help Angelina through it.
"He worries about how much she takes on and tries to do as much as possible to assist her, in this case almost entering the movie with her." Brad - who has no acting part in the film - spends hours discussing the relationship between Daniel and Mariane with Angelina while strumming his mandolin.
The role of Daniel is actually being played by Dan Futterman, who starred in 'Shooting Fish' and 'The Birdcage'.
Angelina sparked a race row after being cast in the mixed-race role in the film.
The casting decision has angered black campaign groups who are furious a white woman has been chosen for the part.
One outraged person wrote on website blacklooks.org: "I had assumed the days when white actors took on roles of black people had long passed."