For the most part, this is simply Janet Leigh toughing it out and trying her damnedest not to blink. Leigh described the process in this interview to Woman's World, and here's an excerpt:
There is also a shot — that seems to go on forever — where the camera is locked on Marion’s dead eye, and Janet somehow never blinks. Not once. Some have suggested that this was a still photo that had sprinkles of water applied to it.
"That is not true," she says. "About three weeks before we shot it, Mr. Hitchcock and I went off to the optometrists.
He wanted me to put in those lenses that would give me a scary look. At the time — remember, we’re talking late 1959/early 1960 — for me to wear those lenses would’ve taken six weeks for my eyes to get used to them. And if I didn't, it could have damaged my eyes.
Mr. Hitchcock said, 'Well, you can’t do that.' I said, 'No, we can’t,' and he replied, 'You’re just going to have to do it on your own.' So I held that look. It’s not a photograph, goddamnit!" She broke into a smile. "I will say it wasn’t easy."
There are a few caveats, though (detailed in this interesting BFI article about this scene):
- During the dissolve from the shower drain to her eye, that is a still frame of the eye (due to technological constraints - they couldn't combine them otherwise);
- You can see that it then goes back to real time (and that it really is Leigh staring unblinkingly) because in the only usable take
they had out of 26(!), her eye twitches the slightest bit. Keep your eyes on her lower eyelid at 0:54-0:57 in the clip below and you might be able to spot it;
- Again, as this was the only usable take, they had to hide the fact that Leigh also took a breath - which they mostly obscured by cutting briefly to the showerhead then back to her, but you can still see her throat contracting a little (from 1:04 onward).