The Oscar winner was shooting the third installment of her family film franchise, Nanny McPhee Returns, in a Hollywood studio next door to where the Clash of the Titans movie was being filmed and she would spend her breaks catching up with fellow Brit Liam Neeson during his downtime on the movie remake.
But Thompson admits she was caught short on one occasion when cameras started rolling on Clash of the Titans - and she ended up hiding behind a prop throne as Neeson portrayed Greek god Zeus opposite Ralph Fiennes and Danny Huston.
She says, "I'm in the Clash of the Titans movie. Liam Neeson was standing there in a lot of gold and silver playing Zeus and I was talking to him, standing there on set in my Nanny McPhee costume...
"The camera was coming around and the first AD (assistant director) was like, 'OK, time to shoot.' And I was standing there talking to Liam Neeson and I said, 'Hadn't I better go?' And he was like, 'No, no, no it's fine, Ralph (Fiennes) is just gonna come up here and he's gonna talk to me.' ...And I said, 'No I don't think it is fine actually' and so I went round and hid behind Danny Huston's thrown. So I am in the shot of Ralph coming into the set of Clash of the Titans, behind Apollo's throne is Nanny McPhee, hiding."
But Thompson admits it was a difficult task - because her movie outfit was so awkwardly shaped.
She adds, "I was trying desperately to sort of (hide). And it's hard to hide in that bloody costume, it's six feet wide!"