Actually, that’s a lie - we start a stopwatch. We want to see how many episodes pass before all that promising diversity starts to die off. Yes, it’s cynical, but we’ve seen it before.
Then one dies. Or two. And/or episodes go by where you realise the POC have been banished to the plot box. Or they have roles that require them to be elsewhere or the other characters have to avoid them. Sometimes they’re there - but always in the background, always on the fringes and you realise that they’ve maybe had 1 line of dialogue in 3 episodes (Hello T-Dog).
As the series continues several of the POC will be completely lost - dead (usually dead), vanished or heaved on the bus. Others will now be silent servants, hanging around the edges, facilitating the plot line of the White people. Few, if any of them will have actual storylines of their own. We realise then we have a case of Pilot Diversity and Minority Decay.
By the time the season finale comes round, or we’re 2 or 3 seasons in, the show will be notably Whiter. Often, the POC left in the cast will not be the same as the ones who started - while many of the White cast have been there since the beginning.
Diverse pilots offer false hope, but they rarely follow through on it. The Walking Dead, Falling Skies, Under the Dome, The 100 and The Last Ship all began with surprisingly diverse casts but as the episodes went by we saw the POC die off or fall into the background (we’ve written posts on The Walking Dead and Fallings Skies) while the White cast become more prominent, survived and kept moving.
Relatedly, we have a trope of shows presenting their minority characters as much more prominently than they are. They may appear on the promotional material, they may even receive high billing in the cast. The marketing for the show will give the impression that the POC cast members are full characters, just a little behind the major characters of the show - or even just not behind at all. Penny Dreadful presented Sembene as a full character in a number of the posters for the show and I kept waiting for him to develop a storyline… which never happened. He was a butler and occasional knife carrier, no more. On Warehouse 13 Leena seemed to be presented as a main character for the cast (it’s actually harder to find a full cast picture that includes Jinks than Leena) but, ultimately, beyond a very few rare moments she was little more than cook and housekeeper for the rest of the cast and rarely took much screen time. Secret Circle tried to present Melissa as a full and equal character with the rest of the circle, but she was clearly an add-on, circling the lives of the actual main characters.
This isn’t a matter of a main POC character not getting as much attention, agency or storylines as the rest of the cast (for example, Bonnie on The Vampire Diaries) so much as it is trying to present and extremely minor or bit parts as full characters. They’re not a poorly handled main character, because there’s not even been the most inept of attempts to make them one - but their presence is emphasised and it’s hard not to think that this is done deliberately to try and convey a sense of diversity that the show doesn’t have.