ON SET WITH... Mark Williams

Mark Williams, 53, best known as Arthur Weasley in the Harry Potter films, plays Brown in a new BBC1 GK Chesterton’s Father series about the crime- fighting Catholic priest. Here, Mark takes us behind the scenes on the show, which was filmed in the Cotswolds...

7 MONDAY

A visit to Wippell’s, the ecclesiastical outfitters, in Westminster to try on my character’s cassock today. I feel things are really coming together now that I’m slipping into the official outfit. Father Brown’s galero – his ceremonial wide-brimmed hat – and his wire-rimmed glasses also help. I reckon I can do my bit to keep production costs down by providing the glasses myself, only to be trumped by the costume designer Giles Gale when I arrive on set in Gloucestershire. He comes up with an even smaller pair which sit perfectly on my nose.

8 TUESDAY

Father Brown rides a very stylish bicycle around the Cotswolds lanes, and it’s fun to be in the saddle. The only tricky part is getting on and off and today I have a few problems as you can’t hitch up a cassock as you can a dress because it’s too tight, so you just have to make the procedure look as natural and elegant as you can. I keep a picture of a young Catholic priest playing football in a cassock in the 50s, when our series is set – unlike the books which were written between 1910 and 1936 – on my dressing room wall to remind myself it doesn’t have to be inhibiting.

9 WEDNESDAY Today we’re filming at a country fete and my 12-year-old daughter Lily has a small role as a visitor – it’s nice for us to share a scene together (right). I get the occasional double-take from members of the public who’re not sure if I’m a real priest. I have to resist the temptation to bless them and confuse them further.

10 THURSDAY

It’s raining and the forecast suggests it will be for some time to come. A great deal of Father Brown is filmed outside to try and make the most of the beautiful Cotswolds scenery, although today it’s largely obscured by mist and drizzle. Still, no time for moping about. There might be some slipping and sliding going on among cast and crew, but we’re a small unit on a tight budget with little margin for error, so we just have to get on with it. And that dark sky can be magically changed to a bright blue one when the programme goes through the editing process...

11 FRIDAY

I realise as I read the script today for the last of the ten stories that have been developed for the series that I’m turning into a bit of a sleuth myself. Puzzles and riddles aren’t normally my thing, though I attempt to do a crossword in my dressing room every day. Now, however, I seem to be developing the knack of spotting the important clues quite early on in each script, and I feel rather pleased with myself when I turn to the final page and find I did know whodunnit! Not that my powers of detection are a patch on Father Brown’s of course...