I think a large part of the problem is that it's simply not possible to do this. The fundamental point of many of Lovecraft's stories is people going mad because they see things that aren't physically possible to see. Probably the easiest story to see this in is The Colour Out Of Space. The whole point is that the titular colour doesn't actually fall anywhere on the visible spectrum. It's not just something different or unusual, it's something that should be completely impossible for humans to experience, yet they do anyway. How can you show a colour that doesn't actually exist on film?
Well, okay - I'll concede that
Colour Out of Space might be unfilmable as a direct translation from the story as written, but purely for obvious technical reasons. Other films also wouldn't work as
direct translations either - for instance,
In the Walls of Eryx simply doesn't work because it's public knowledge that Venus does not have forests, or natives, nor is there any kind of special suit people can wear to make walking on its surface survivable.
From Beyond is out - there's consumer electronics capable of seeing ultraviolet light and we all are aware by now that there aren't strictly-ultraviolet-colored creatures flitting about through the air all around us.
However, I think there may be fair ways of changing these stories only just enough to get around these problems without really affecting the points, hooks, themes, or flavors of the stories. For instance,
Eryx - this story can happen on any planet, some place that does have the forests and native aliens; there's no story-essential reason it HAS to be Venus.
From Beyond - quite simple; just have the machine make its user able to "see", say, radio - or microwave light, rather than ultraviolet. The story winds up the same - it's technically "light", that we can't see, so we have no idea what it would look like if we suddenly had that power. Nothing that is common public knowledge precludes the existence of microwave-energy creatures! It even provides a somewhat plausible mechanism by which the creatures can kill human victims...
In similar ways, I don't think movies featuring "indescribable" things are of themselves necessarily unfilmable. As with the above examples, there are workarounds, ways to portray certain scenes and details to remain true to the stories, but which sidestep these little problems. In the case of the indescribable monsters that drive people crazy? The most obvious answer - although it seems that by far
most directors simply CANNOT bring themselves to do it - is simply not show the monster! Ever! No big "scary" reveal at the end. Maybe little snatches of incomplete detail here or there - think the bulk of
Cloverfield before the "money shot". Maybe even less than that. A person hears an indescribable voice? Don't let the audience hear the voice - let a character hear it and relay the message (and point out the voice is indescribable). Even as written,
The Statement of Randolph Carter has Carter tell us what the voice on the phone told him, rather than having us be there with him when he hears it. Earthborn's idea regarding using "impossible colors" for
Colour Out of Space is highly intriguing! But it can also be done simply by not showing us the "color", although we can see the characters seeing it. The actual color itself doesn't play a tremendous part in the story anyway, compared with say the gray creeping "blasted heath" that the object seems to spawn.
I recall seeing an episode of some Twilight-Zone-like TV show once, though for the life of me I can't remember its name; but in this particular story someone has somehow discovered The Secret Of Life - maybe on a trip to some foreign land? I forget - and he begins to tell others about it, but the problem is that every person who gets told goes crazy, sometimes violently. Eventually one man figures out the connection, and rushes home to warn a family member to turn off the radio as the guy with the secret prepares to announce it on a radio talk show. We stay outside as he rushes into the house, unfortunately evidently just in time to hear the Secret of Life.
We don't hear it; all we hear is the protagonist's mad screams.
That's the way to do it, methinks.
