When a theater troupe visits Hamlet's castle, he makes some... rather pointed changes to their show, which now includes a murder much like the one his uncle performed. Why didn't anyone else (especially his uncle) figure out (or, at least suspect) such an obvious ploy? And why didn't anyone find Claudius's bizarre reaction to the addition suspicious?
Well... who says they didn't? We see the play mostly from Hamlet's point of view. He has few allies, and deliberately pushes people away. For all we know, lots of other people were suspicious of the guy who started screaming when presented with a situation not unlike the circumstances of his rise to power. They just didn't do anything about it, at least not in Hamlet's field of view.
Of course, if they were plotting something, they'd hardly tell Hamlet, would they? He's been acting nuts lately.
There is much to be made of Gertrude's reaction. The play is ambiguous about her knowledge of the situation. I've seen it presented with everything from her being a complete patsy, to a woman making the best of a very precarious situation, to a co-conspirator in the murder. It will be hard for a live audience to look at her at the moment Claudius is yelling for the lights, but the actor should certainly be making choices at that instant about what she is learning. A film director, with better control over the audience's point of view, can help direct their attention to her choices.
As for everybody else, there is one nice bit of text that helps guide how to play the rest of the court:
The king doth wake to-night and takes his rouse,
Keeps wassail, and the swaggering up-spring reels;
And, as he drains his draughts of Rhenish down,
The kettle-drum and trumpet thus bray out
The triumph of his pledge.
it is a custom
More honour'd in the breach than the observance.
This heavy-headed revel east and west
Makes us traduced and tax'd of other nations:
They clepe us drunkards, and with swinish phrase
Soil our addition
In other words... the king parties hard, and the rest of the court with him. So it would be an easy choice to make that nobody does really connect Claudius' outburst with a guilty conscience. They're all having a really good time, and maybe they think he's just drunk. Or perhaps simply don't care.
They do all seem pretty out of it. Nobody cares that Hamlet was passed over for kingship. When Fortinbras comes in, there's nobody to tell him that they don't want a Norwegian (with a French name for some reason) to replace the Danish king (with a Roman name). The entire state of Denmark appears pretty rotten, not just the guy at the top.
Hamlet arranges a scene so the players do not think R&G are too close to him, so they won't tell R&G about the changes:
Your hands. The appurtenance
of welcome is fashion and ceremony.
Let me comply with you in this garb,
lest my extent to the players
Which must show fairly outwards.
Should more appear like entertainment
Hamlet similarly berates Polonius in front of the players:
Follow that lord,
and look you mock him not [hint-hint].
During the play Claudius is put off his guard by Hamlet directing his first question to Gertrude:
Madam, how like you this play?
Claudius thinks "So this is all he's up to, ridiculing our hasty marriage?"
Shakespeare has your question covered.