I wondered if in some bonkers self-justifying way, the MPs felt they were underpaid, so I worked out how much their net income was.

Then I started comparing this with the take-home pay of other professions.

Monica and Bryan Adams and their daughters are one of the families featured

I realised we all make big (often class-based) assumptions about how much other people - friends, relatives, colleagues - earn, but that actually, because money is such a taboo subject, we rarely know the truth.

As a film-maker, I felt that a TV audience would probably be as curious as I was to know how much money people actually take home, as well as to find out what people choose to spend their money on.

I had also noticed that many people (myself included!) often say they can't "afford" something (cinema tickets, childcare, smartphones etc) when what they actually mean is that it's not a priority for them and they choose to spend their money on something else (holidays, pension plans, eating out etc).

I chose the figure of £40,000 to base the programme on because it turned out to be the average net household income in homes where two adults are working - but also because it's a large enough income that I would be able to talk to people who were in the fortunate position of being able to make choices about how to spend their money.

The search for contributors with a net household income of £40,000 took a long time and involved many imaginative strategies.

I had a couple of assistant producers working with me - and between us, we must have approached literally hundreds of groups, organisations and associations.

We also leafleted all sorts of different locations around the country (shopping centres, cafes, factories, parks etc) and even stopped quite a lot of people in the street!

We were looking for people from all over the UK, so from a research point of view, it was a huge and sometimes daunting task.

The people who volunteered to take part were only ever going to be people who were willing to discuss their finances.

But actually, in the course of making all three programmes in the Money series, by and large what I found was that many people are actually willing to talk about money honestly, especially if they don't have ridiculous amounts of it.

If you're spending all your earnings on paying your bills and supporting your family, you're unlikely to have huge embarrassing secrets.

Debt seemed to be the most shame-inducing or difficult subject to speak openly about.

What I hadn't anticipated was the powerful emotional moments in the film. I hadn't expected so many people to burst into tears!

But the privilege of making films like these is that you get to talk to people about things they feel passionately about in their lives, and the joy of documentary-making is that people are always full of surprises.

Vanessa Engle developed, produced and directed Forty Grand.

Comments made by writers on the BBC TV blog are their own opinions and not necessarily those of the BBC.