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Whenever Dexter has his victim on the slab ready to be killed, he cuts them with a surgical knife and takes a blood sample. Obviously the knife is fake and the blood is fake but quite often you will see the knife touch the skin and the fake blood comes out, emulating a cut on the cheek.

How is this effect done?

  • 1
    There are chemicals which when touched with some substances, give red color and are harmless to the skin. – ABcDexter Nov 30 '17 at 9:44
  • 2
    Possible duplicate of How slasher scenes get shot? – Luciano Nov 30 '17 at 9:52
  • 1
    Also worth taking a loom at this which talks about how some effects on Dexter are done – Longshanks Nov 30 '17 at 18:30
  • I remember a scene from NCIS where Abby 'open her throat' with one of those knife that 'draw blood' to get Gibbs attention (and to prove that the murder in the video was not a murder)... – dna May 21 '18 at 7:00
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It's a hollow knife - fake blood is pumped through and ejected when the fake blade touches the skin.

Or the blood is routed along the (very blunt) blade on the side away from the camera. If you're a right-handed actor, you can use this knife to slit someone's throat without the pipework and bladder being visible to the camera.

enter image description here Source

This is the most effective/realistic method.

Or you can use prosthetics and have a real knife slice into a squib, but this can be pretty messy and you need a new set-up for each take.

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So this is actually really interesting and not far from what @Snow said.

Before we get started, all the following information is coming from Joshua Meltzer, the master prop designer for Dexter.


Blood

Image result for dexter blood scene

Meltzer actually created his own blood formula for the show, consisting of maple syrup, food coloring, peppermint oil, and Dawn dish soap. Maple Syrup adds the thicker texture and cleans easier than corn syrup, peppermint oil repels the insects that are attracted to the syrup, and the Dawn soap makes the fake blood easier to clean out of clothes.


Blood Samples

character taking a sample

So when Dexter collects the blood samples, the blood is the maple syrup blend, but when we see the case of samples the dried blood is actually furniture dye.

Image result for dexter blood samples


Cuts

Now, finally to your question.

Surprise! Thanks to the magic of props, the scalpel Dex uses has a "rig"; which means it&'s a dull blade rigged with a blood pool on one side. So when they film, the side filled with blood goes to the off camera side, and when it comes across the cheek, Michael presses and forces it out and onto the cheek.The blood that oozes out is that same maple syrup mixture.

In these scenes, Dexter uses a rigged knife, which is essentially a dull-blade knife that has a "blood pool" on one side (the knife below is the one used on set).

Rigged knife

They have knives with blood pools on different sides to get cuts from varying angles.

Meltzer showed me this knife in the prop room. TV magic at its finest!

Meltzer was actually showing off this knife to a reporter.


I hope this was helpful :)

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