I recently found out something interesting about the film Erin Brokovich starring Julia Roberts as Erin Brokovich and Albert Finney as Ed Masry (her boss).
About 10 minutes into the film, Erin Brokovich (Julia Roberts) takes her kids to a diner.
The name of the waitress who takes their order is Julia.
So Erin Brokovich played by Julia Roberts gives her order to Julia the waitress who was actually played by the real Erin Brokovich.
To complete the scene, the older guy sat behind Erin was played by the real Ed Masry (her boss)
There is something similar in La Bamba. Near the end of the film there is a family gathering and the old lady sitting on the end of the sofa is Richie Valens' mother in real life. She passed away a few months after the film's release but was honoured to have been asked to be in the film and loved the tribute to her son.