She thought it would be about Jesus. She thought that the film - like the festival we're recovering from - was supposed to be about the saviour of the world.
"Once we get away from talking about Jesus," Sister Wendy Beckett told the man who had persuaded her to make the film about "the art of the gospel" on Christmas Day, "I'm in darkness".
She might have thought she was, but she wasn't.
She might have thought that she only knew how to talk about God and art.
But when this woman, who has been a nun for 67 years, started talking to the director, Randall Wright, about God and art, and the paintings she had chosen for this Arena documentary - shown after the Queen and before Top Gear - it was clear she was talking about lots of other things, too.
When she talked about Jesus, she made you think about how unusual it was to hear people talk about Jesus and how the stories that many of us grew up with now seem almost to have disappeared.
When Sister Wendy started writing books about art and was asked to go on TV, she had never watched a programme, or switched on a TV.
She doesn't have a computer. She dictates her books to a priest. But when she talks about Jesus, she sounds like a very wise woman who is letting us see her heart.
When she talked about the experience she had as a child, of feeling the presence of something she thought must be God, those of us who don't believe in God could only wonder about the feelings, the experiences, of those who do.
When she said that "one of the things prayer will do is show you yourself" and that "that's something most of us will go to a lot of trouble to avoid", even those of us who don't believe in God couldn't help wondering if she was right.
When she said that "now nearly everybody can live their whole lives being entertained" and that "it means that you're never in contact with what you are", we couldn't help thinking about how we usually thought about what we did, whom we talked to and what we had to do next; tending not to give much thought at all to who we were.
And when the programme had a whole minute of silence, it was a shock.
In that silence, we saw the pictures she had talked about again.
It made us think about what she'd just said - that "great art challenges you" - and also what she'd said on Desert Island Discs last week - that great art is "demanding" and that you have to be "totally there".
It made us wonder how often we were ever 'totally' anywhere, or how often we ever gave our full attention to anything and what pleasures we might be missing out on if we didn't and weren't.
It made us think of the poet John Clare, who said that, "after the hustling world is broken off", there "is a charm in solitude that cheers", but also that the world he thought was "hustling" in 1837 was much, much more "hustling" now.
And it made even those of us who don't believe in a God think that "goodwill on earth" was an awful lot to aim for, but that there was quite a lot to be said for a bit more peace.
And that if we were to try to find a bit more peace and quiet and silence, then maybe we, like this passionate nun who lives in a hut as a hermit, but loves the world so much, might see more of what we have - and who we are.
Sister Wendy knows where she will be buried. She will die, she says, thanking God for "allowing" her a life of such "unimaginable" happiness. "Lucky me," she says.
And lucky us, for this lesson in joy.