It presents itself as a smile and seems to be triggered by mention of Hampden, something which has come up a lot ever since St Mirren booked their place in the final. Thompson did his best to suppress it yesterday – he muttered something about a game against Dundee United – but it was not long before his lips were twitching again.
Danny Lennon, his manager, did his best to mop up by insisting his players would show "the right application" against United today. It would take an uncharacteristic loss of professionalism for Thompson to show anything other than that, but yesterday it still seemed more pressing for the striker to discuss the final with Hearts.
He let his audience in on how he was handling the pressure, too. Talk of "head space" and having "a few powerful visions" might have suggested Thompson was ready to lie down on a couch and revisit some unresolved childhood trauma – he is a boyhood St Mirren fan, after all – but his insight was informed by exercises he has picked up from sports pyschologists.
"You would be lying if you said you weren't thinking about it," said the 34-year-old, whose side will be without Lee Mair and Jim Goodwin. "I think about it on a daily basis. It is consuming most of my head space and has been for weeks.
"I have been visualising plenty of things and they are all positive. Visualising is an important part of preparation and in my head I have got quite a few powerful visions. A lot of people were talking about visualisation during the Olympics and it is a tool I try to use."
United will hope that such visions obscure his view in Paisley today. The Tannadice side defeated their city rivals in the William Hill Scottish Cup quarter-final last weekend, while there is also the matter of breaking into the top six before the split. "It's all about winning at this stage," manager Jackie McNamara said. "We'll have to be prepared to dig deep against St Mirren as they are a tough team to play against. The bottom line is trying to get the three points."