Usually, when producers try to talk people into appearing in observational documentaries, they say that after a while 'you won't even know we're here'. And, of course, the producer hopes that more than anyone, because it means being able to film people behaving naturally.
In my experience, hanging around for ages helps achieve this, but the most important factor is whether the people being filmed are doing something they find more interesting or dramatic than being filmed.
Which brings me to last night's Inside John Lewis documentary on BBC2, the second in a three-part series. I'm afraid there were times when I felt - perhaps because the series is three hours long - we were looking at moments where being filmed was more interesting to the participants than what they were supposed to be doing. (As when the new recruit in Wales turns to camera, having criticised the John Lewis wine he was tasting, and jokily says something nice about it instead "for the documentary".)
And there was a bigger problem. This was the story of a rather eccentric business adapting itself to practices that are common in more 'normal' retailers. A pity, really, both for John Lewis and for the programme-makers.
So it would not be surprising if these three hours have less to offer than BBC2's previous, entertaining record of life inside John Lewis, the 50-minute Modern Times film called The Partners, shown in 1995.
But, hey, what do I know? The audience loved it: 2.8 million last night (thrashing Ramsey's Kitchen Nightmares USA on Channel 4: 1.7 million), and 3 million viewers in the first week. As they say at John Lewis, the customer is always right.
