In 2007 I took a two-year break from shooting the BBC series Hustle, in which I played a big-time con artist. I’d been making the series for four years and I was afraid of being typecast. For a change I got involved in a series called Bonekickers, about a team of archaeologists.

In this photo, from left to right, you can see Julie Graham, me, Hugh Bonneville and Gugu Mbatha-Raw. The four of us had formed a little gang: the crew changed and the director changed but we were the hard core. We worked, ate and even went shopping together.

It had been a long day of filming, and we were shooting a scene on the muddy shoreline of the Bristol Channel. In the background you can see the M4 heading over to Wales. We had to discover certain artefacts buried deep in the mud.

It looked like quite a straightforward scene when it aired on television, but what the viewer doesn’t see is the camera and lighting crews spending all day lugging heavy equipment through a foot of mud in order to place the camera in a different position for every shot.

They did it all without complaint, making sure the camera gear was operational and mud free, ready for action, take after take. Things got a little tense on set.

We were, as always, up against the clock. But out of the corner of my eye I could see Hugh Bonneville gearing up for trouble.

He had been flicking tiny pieces of mud at us during the scene, and evidently he was not about to let the day end without having a little fun.

In waders, bent double, we delivered our final lines, and the director shouted, ‘Cut!’ Hugh grabbed a handful of mud and waited.

I knew what he was about to do, but I couldn’t move until the scene was finished. The first AD finished chatting with the director, and shouted, ‘Scene complete!’

It was the red rag Hugh needed: he arched back and launched the mud in Julie Graham’s direction – and missed.

She tried to run, but the mud was too deep. I tried lobbing a handful at her moving target but missed too. Gugu – the youngest member of our team and the most conscientious – had a look on her face that suggested she’d rather not be involved.

Which prompted Graham to hit her with a fistful of mud, square in the back.

Bonneville rounded on me and flung another handful. This time it was a direct hit. Suddenly, like a bunch of tabloid newspapers, we were slinging mud at anyone and everyone.

I laughed so much it hurt. We needed a photo, and gathered in a group to mark the moment.

The final episode of Undercover, starring Adrian Lester, is on BBC One, Sunday at 9pm.