"My son was sitting on the other side [of me]," the actor continued.
"And it was kind of unfortunate, because once it starts getting exciting, about 45 minutes in, he was so carried away with things — he was about 10 years old at this point — he started to fart unreasonably."
While the 76-year-old confessed the flatulence "wasn't that audible", it was "very, very (smelly)".
"There was sort of a draft through the cinema, and it was all drifting in the princess’ direction," the actor added, confessing he was "sweating like crazy" thinking Diana would attribute the wind to him rather than his son.
Regardless, as any good father would, Sam didn't rat out his young-in, who is now in his 40s.
The actor's story comes five months after he shared an update about his ongoing treatment for blood cancer.
In October last year, Sam confirmed that his chemotherapy treatment had failed and he had instead turned to a rare anti-cancer drug. He is now in remission.
"It's like going 10 rounds with a boxer but it's keeping me alive, and being alive is infinitely preferable to the alternative," the actor said on Australian Story.
"But I've also got to be realistic. One doctor said to me, 'This stuff will stop working one day too', so I'm prepared for that.
"I'm not in any way frightened of dying, but I would be annoyed because there are things I want to do," he added.