More of a travelogue than a documentary, Perkins begins in the border town of Tijuana, where she finds tequila-fuelled parties sit uneasily alongside the scale of asylum seekers’ suffering

Watching Sue Perkins present a programme always brings to mind the moment in Blackadder when Edmund, in financial straits, is showing prospective buyers around his home. “You’ve really worked out your banter, haven’t you?” says one of them. “No, not really,” replies Blackadder. “This is a different thing – it’s spontaneous, and it’s called wit.”

Wit is Perkins’ USP. All presenters have warmth and intelligence, though both can vary in degree and kind, and in the proportions in which they are blended. But it is Perkins’ ability to think on her feet – and, I suppose, the willingness of her editors to keep it in and not flatten her into traditional affectlessness – that marks her (and the likes of Grayson Perry and Paul O’Grady when he lets rip) out. It adds zest to proceedings. This is always welcome, even when – as with last night’s opening episode of the two-part Sue Perkins: Along the US-Mexico Border (BBC One) – the programme’s subject matter is notably colourful stuff on its own.