With NTR I am referring to it's classical meaning, namely that the main character's Love Interest / partner is (sexually) stolen by someone.
First of all, Netorare works best if the player is not aware that it is going to happen in the story. However, if the game's goal is to humiliate the main character, this is less important, because it's not about invoking jealousy, but to degrade the MC.
NTR is supposed to give a feeling of jealousy and powerlessness and as with many (strong) emotions, the element of surprise works as an intensifier.
Comedy is a good example, one of the most effective ways is to use the element of surprise, or saying something unexpected. This can provide laughs even in rather unfunny moments.
As many players hate NTR and therefore the NTR tag is needed, it is not easy to implement surprise NTR.
In my opinion, there are 3 important NTR set ups.
1. Sledgehammer: Simple but effective. Here the element of surprise is of most importance.
An example: the main character opens carelessly a door and witnesses his moaning girlfriend being fucked hard from behind.
Optimally, the player is overwhelmed with a wave of (unpleasant) emotions. Of course, there must be no indication of this event might happen.
2. Slow Burn: The most difficult to implement, because you have to walk a thin line. The player must not find out about the impending NTR too early, it has to build up slowly, perhaps giving hints here and there. What's important is an emerging uncertainty as to whether this could be NTR or not until it's too late.
An example: The MC's girlfriend typical best male friend who gets along great with her and there is also a certain physical closeness (without the typical but stupid hand on ass shock moment), but the girlfriend assures that they are only friends and may even get angry if the MC gets jealous.
3. Failed hero: The player must have the feeling that he can easily avert an impending possible NTR situation, but this is just an emotional trap for the player (Yes, in a way it's kind of a combination of 1 and 2).
An example: The main character learns that his girlfriend has been dragged to a motel and is about to get raped. Of course the MC rushes over in time and pulls open what appears to be the correct door, only to find himself in an empty/wrong room. The scene changes to the right room, where the perpetrator penetrates the girlfriend at that very moment.
The player wasn't fully expecting NTR because he assumed he was going to save the victim.
Bonus: Guilt: And here is something that makes NTR so ... particularly evil. Works best with 2 and especially 3. The scenes leading to the (presumptive) NTR are set up in such a way that the scenes exites the player, and when the NTR happens the player is too horny not to be aroused by the sex and then feels guilty.
A rather simple example would be an erotic foreplay sex scene and as the the penetration happends it shows that it's not the player.
I'd like to have given better examples, but English is not my native language and it has taken me enough time to write this at it is.