I am going to add my two cents, half a year later, because I haven't seen this said out right yet:
You can't create tension by changing the conversation, not really.
If there isn't tension in the scene, the situation is the problem, not the conversation.
Tension requires two things and two things only
- A character your readers care about
- Uncertainty in that character's fate
To drive this point home, there is a really creative and suspenseful "gun to your head scene" from the movie Sonatine, in which the conversation involved is 'calm'. I am willing to bet that I can show you two short clips from the film, one to get you to care about the characters, and one to make you uncertain about their fate, and you will feel suspense even without seeing the rest of the film.
Just a little context, the characters are whats left of a yakuza gang, in hiding due to a gang war gone wrong, and you find out near the beginning of the film that almost everyone they cared about recently died. The older character in the clips is the leader of the gang.
Here is the first clip of them goofing off on the beach to get you to care about the characters.
Here is the second clip to get you uncertain about their fate.
There is hardly any dialogue; the suspense doesn't come from the conversation. In reality, there isn't even a real threat. The suspense comes from the fact that we care about the characters and we don't know whats going to happen.
So how can you apply this to your scene? Make sure your readers care about the character who has the gun pointed at them, and then make sure that your readers are uncertain about whether the gun will go off somehow. Here are some ideas.
You could do this by bringing up the question of whether or not the
boss has it in him to kill way earlier in the novel, and throughout.
Maybe he is guilty about killing her friend, and isn't sure if he is
ready to kill again.
Maybe you could show, somehow, that the gun is broken and might go
off at any moment even if the boss doesn't pull the trigger, and may
not go off even if he does, showing this many scenes earlier so it
doesn't feel artificial, and maybe making the characters unaware.
You could make the boss psychotic; maybe he flips a coin or plays
Russian roulette instead of killing her out right. This one is a bit
cliche, but it can clearly work if Sonatine, The
Dark Knight, and Bang! Bang! You're Dead are anything to go
The point is, tension doesn't come from a well written and believable conversation; tension comes from an uncertain and believable situation.
Don't get me wrong, a well written conversation can do a lot for a scene like this. A conversation can make the situation sadder, or funnier, or creepier, or more disgusting, or happier, or make us hate the boss more, or love the boss more, or love the mc more, but it can't in and of itself make it more tense.