In “Castle’s” fourth episode, “Hell Hath No Fury,” Castle and Beckett investigate the suspicious death of a city politician running for re-election. The red herrings fly like a busy day at Pike Place Market: there’s the campaign manager, the opponent, the frustrated real estate developer, the scorned call girl and the grieving wife. Given the title of this episode, you can probably guess who’s guilty—thankfully, it’s not that clear cut.

Again, the beauty of “Castle” isn’t the case, it’s the rapport between Castle and Beckett and the ways in which Castle’s knowledge of mystery as a genre and story form provides insights into the cases he works on. And, in their fourth week, the actors and creative team seem to have hit on a strong formula—one that not only highlights the wise ass that Castle is, but the beauty of his push-pull relationship with Beckett.

However, I’m not sure how much mileage the show’s going to get out of it, and it seems that maybe what’s needed is development of their secondary characters. Beckett’s colleagues including Captain Montgomery, Detectives Santiago and Ryan, and Medical Examiner Lanie Parish, need more screen time. Ruben Santiago-Hudson (Montgomery) and Tamala Jones (Parish) have guest starred on a variety of primetime shows for years, paying their dues and deserve storylines of their own. And while I like Jones’ best friend relationship with Beckett, I think that’s a stereotype that could get old fast. So far, nothing new has been introduced in the girlfriend archetype and I’d really like to see something different.

The continued presence of Castle’s snarky and precocious daughter, Alexis and his out of control mother, are welcome. Still, I’d like to see something new here too. The show can only tread on the standard procedural formula for so long, because it’s trying to do something else. Eventually, viewers are going to want that to pay off.

If the show had presented like a derivation of “Law & Order,” where we don’t care so much about the detectives or the lawyers but about how they investigate and prosecute the case, then we’d be watching for the cases. But we’re not. We’re watching for Castle and Beckett and their relationship. In order for it not to get old, it needs to push beyond the normal “she’s annoyed, but he’s so cute, it doesn’t matter.” If we’re to believe that Beckett is a smart, savvy detective and a strong, independent woman, then she can’t be easily (or ever) swayed by Castle’s wiseass ways. Their relationship needs to develop on a deeper level in order for it to be satisfying.

I’m still enjoying “Castle” and I don’t see a reason that I’ll stop. But adding it to my must-see TV list is going to take more than wisecracks and charm. It’s going to take relationships of substance and layered character development.

But for now, I’m enjoying it, case by case.