Huh. You’ve just made me wonder about something new: when Columbo produces the box and starts talking, the suspect’s whole demeanor of course changes; Columbo yammers on and on for a while, and the suspect reacts more and more like he’s guilty and less and less like a blissfully unaware innocent. And then Columbo gets ready to open the box.
Does Columbo now have more reason to believe he’s about to make the guy fear for his life than before he revealed the unopened box?
But that’s not your original argument, which as I understand it, is “is it a crime to threaten a murder suspect with death, if it is only perceived as a threat at all if the suspect is actually guilty”. That is an interesting question. I would agree that a threat of death can be considered assault, but it would only be possible to be considered assault if the person was guilty in the cigar box scenario. And even then, as @HMS_Irruncible points out, it’s a veiled threat at that, because the suspect does not know that Colombo is threatening him, even though he is guilty.
This thread is getting duck-gun’ silly 
This is not what I set up in the hypothetical. You are making the exactly, precisely the same mistake in both cases. You are assuming that the person doing the ruse knows more than they’ve actually demonstrated they know.
The duck-robber doesn’t know the cashier is afraid of the duck (or at least, I purposely left that fact unknown). He’s just trying something for reasons nobody knows, which I haven’t clarified. You are assuming he has knowledge that I purposely left out, because I wanted to show you what it looks like when it’s not Columbo, but just a weird guy who acts like he knows things.
It’s the same with Columbo. He’s trying a certain ruse for reasons that nobody really knows. Because it’s Columbo, we assume he knows exactly what he’s doing, but based off what I know of the episode, he just tried an odd hunch for some reason.
Ah, you youngsters. 
California had the death penalty until 1972, and used it.
Maybe, but he still doesn’t know. Suspects act weird all the time for their own reasons. Maybe there’s something else scary about the cigar box, other than what Columbo thought. He needs the suspect to speak or act specifically about the box.
How dare you! I’m not young!
Just dumb 
I apologize if I didn’t make it clear, but:, my argument is that (a) it’s a crime to threaten someone with death, regardless of whether they’re a murder suspect and regardless of whether only a guilty person would perceive it as a threat; and that, as a subset of that, (b) it’s a crime to threaten someone with death even if they’re a murder suspect and even if they’d only perceive it as a threat if they’re guilty.
Granted, the second part is the case we’re discussing — but I figured all along that the first part, which Includes the second, is true. So when you say “I would agree that a threat = assault, but it would only be assault if the person was guilty in the cigar box scenario” — well, I’d say that runs afoul of the first part and the second part.
See, I’d argue that he does know that Columbo is threatening him; he just doesn’t know whether Columbo intends to threaten him. Which is, as ever, a pretty fine hair to split — but it’s arguably a pretty important one.
But he doesn’t need to know. The question is what — without perfect knowledge — he intends. And he has more evidence now than he did before.
Clearly you want this to be the whole question, but it’s simply not.
It’s only a threat if he has reasonable belief that his actions will cause the other person to fear harm. Columbo does not have that reasonable belief. He has unreasoned suspicions. He is developing the belief by prompting the suspect to speak or act.
After the suspect confirms he believes the box could harm him, then further use of the box would be a threat. After the suspect confirms he believes the (empty) box is a bomb, Columbo can’t legally say “tell me who helped you, or I’m opening the box.” That’s clearly a threat.
Without that confirmation though, Columbo doesn’t intend to threaten, he only intends to find out if the suspect feels threatened.
This seems to be taking it too far; I agree that he doesn’t have 100% knowledge, but I’d disagree that he only has “unreasoned suspicions.” I’d argue that he has reasoned suspicion even before he reveals the box — and more after he reveals it — and that, when he opens the box, he either has a reasonable belief that it’ll work (while admitting he might be wrong, because sometimes his traps don’t work) or a reasonable belief that it won’t (while figuring that he could be wrong, such that it might work).
Analogous to an officer asking “If I asked your buddy would he tell me the same story?” - which ideally is intended to make a guilty person feel worried but an innocent person not.
Since Law & Order: Criminal Intent’s Robert Goren has been mentioned a few times, I was reminded of the episode (“Person of Interest”) with him interrogating archnemesis Nicole Wallace that’s almost the opposite of threating a guilty suspect with a cigar box that he thinks is deadly–threatening Wallace with deadly anthrax exposure that she’s been vaccinated against but her assumed identity has not (and therefore she doesn’t react the way a unvaccinated person would upon exposure). Wallace knows the threat is not deadly to her but she should have acted as it was.
After deducing Nicole’s plan, Goren and Eames arrest her stealing the anthrax. During her interrogation, Nicole once again tries to get inside Goren’s head, while also mocking Eames, who was a pregnancy surrogate for her sister’s child, for having “eggs ripe and ready for hire”. Goren tells Nicole that she has become the abuser of the “sparkling little girl” she once was, and that she keeps trying to destroy him because she knows, deep down, that he is right about her. He then says that she has been exposed to anthrax, to which she replies that she has been vaccinated; this finally proves that she is not Elizabeth Hitchens, who had no record of being vaccinated against anthrax. As Garen arrests her for murder, she warns him that she is not finished with him.
He does not think Columbo intends to harm him. There’s no intent there.
But, as far as I can tell, that’s not the right question: if he believes that Columbo is about to harm him, and Columbo intends to cause that belief, then — to the best of my knowledge — it doesn’t matter whether he thinks Columbo intends it.
He has no reason to believe that Columbo intends to harm him. Your playing with the words doesn’t change that. He has no knowledge of any intent from Columbo to do harm to anyone. No reasonable person would think Columbo intends to harm them and himself in this situation. Only a person who put a bomb in that box would think anything like that.
What the actual fuck?
You’re making my point. I’m saying he doesn’t believe Columbo intends to harm him. I’m going beyond saying he ‘has no reason to believe’ it — and beyond saying that he ‘has no knowledge of any intent from Columbo’ of it — to flatly state that he does not, in fact, believe that Columbo intends to harm him.
If Columbo harms him - in the cigar box scenario - Columbo also harms himself. Therefore the suspect was very clear that Columbo didn’t know what he was doing (with the box of cigars).
On what basis would you argue this? What specific reasons does Columbo have to believe that the suspect placed explosives in a cigar box? Not suspicions or feelings, what facts does he possess.
Unless I’m overlooking some details about the episode, Columbo has no reason to believe the suspect will fear a certain type of cigar box. He’s trying to find out if the suspect does exhibit that fear.
Obviously he’s trying it because he thinks it’s in the realm of possibility. But he doesn’t actually have reasons for that belief. He’s trying to develop support for that belief. If he already had it, he wouldn’t need the ruse.
Edited: I feel like I must be overlooking some detail about the episode, because it’s not at all clear why Columbo thinks the foul play was specifically around an explosive in a certain type of cigar box, which obviously would’ve been destroyed by the explosion, leaving no evidence. It’s hard to imagine that big of a plot hole, there’s got to be more diegetic detail around that, but nobody’s mentioned it so far.
IIRC - in one (or more) scenes, Columbo visits the suspect’s lab and starts putting it together.
I think it’s entirely possible to believe — reasonably, and with some support — that a guy committed a crime, while also believing that you don’t yet have enough evidence to convict him. It’s always struck me as utterly consistent when someone has explained that, yeah, he’s probably guilty, but the prosecutor needs to clear a higher bar than More-Likely-Than-Not.
And so I figure he can try to develop yet more support for a given belief even if he already figures it’s more likely than not.