I doubt there are many people here who haven't seen it, but for them this is a SPOILER.

Everyone knows that Dexter and Norman plan the murder itself together, but when it comes to something else, I wonder how much they cooperate.

I mean all those accusations they make against each other to Columbo to confuse him. Especially the casino scene. Norman looks genuinely surprised to see Dexter and Columbo there, so I wonder whether they planned THAT together or Dexter came up with it on his own.

Obviously neither one wants Columbo to arrest the other - because then HE'D be caught - but it's easy to think that each one enjoys TEARING DOWN the other to Columbo, along with confusing him. And again, maybe it's something they don't plan together in detail.
Again, one reason I wonder about that is how convincing it is in the casino scene when Dexter smiles and waves at Norman, and Norman glares at him. Maybe it's all just "part of the act," but who knows.

Grant: I always wondered why they were always turning on each other, in real life that would never happen. My only explanation is the writers wanted to keep the audience unaware they were BOTH involved, after all w did see who actually did it.

According to legend that scene was completely improvised and they just kept it in.

Next time I'll count how many times Columbo says "Uuhh"...