Clues

Evaluating the clues you found and making sense of them is fun.
Having a hard time finding them is not.

Some people will disagree, but this is what i experienced, most of the time, as a player or GM.
Players tend to get frustrated and bored if they don't know where to look and fail advancing the story.

So make your clue gathering easy and obvious. Be generous when handing them out and don't make it unnecessarily difficult for your players to find them.

It does not mean they should not be working for them. Of course they'll have to talk to some NPC's, search places and come up with ideas. But try rewarding any idea they come up with.

It leads me to the "floating clues" as they are explained in Trail of Cthulhu.
It's a very clever concept that has helped me a lot since i realized it.
Some clues, as many as you can afford, should not be locked down in a specific place, event or NPC. You should keep them in the back of your mind or on your notes, and place them, during the game, wherever your players will be looking.

Example: The adventure states that the widow Smith will tell the players that her deceased husband joined the expedition to Africa in 1923. If the players don't talk to her, they will never find out. If this is an essential clue, players will probably be stuck until they have the idea to talk to her. It also gives them the feeling of being railroaded.

If you think about it, there are many possible sources your players could get this information from. They might read it in the newspaper archives, they might learn it from one of Pr.Smiths students, or one of the his co-workers at the university, or they might find a letter in Pr. Smiths desk, a picture on his wall. It's not important where they will get the information from. So make it easy for your players. Reward their ideas by giving them the clues they need to advance the story wherever they cared to look.
This is easier if you keep your clues floating and not locked down somewhere.

It's a good way to avoid looking railroady. Even if the adventure actually is. The players won't realize it because wherever they are investigating, they will be able to advance the story. You will be able to steer them where you wanted them to go in the first place by still giving them the clues that will take them to the next part.

Horror in roleplaying games

Scaring your players when they are sitting around your table with some snacks in front of them will obviously be very hard.

You can get them nervous though, giving them the chills and having them doubt their actions.

Fortunately the player's imagination will do most of the work for you. All you have to do is getting their imagination going. 

What is it we are the most scared of? I would say the unknown, thinking there is something without knowing what it is, where it is and what it wants. 

This is mainly how you will get your players to experience horror in a roleplaying game.

Don't just throw your critters at your players. Instead build up the tension. Start by giving them hints, noises, irrefutable proof that there is something nasty stalking them, or hiding in a dark cave or basement they need to explore. Keep everything mysterious.

Let them react and bait them to find out more. In the mean time, their imagination is running wild, trying to make sense of what is happening and filling the missing links with horrible stuff
.

If you go with "While investigating the corpse washed up on the shore, a deep one jumps out of the river and attacks you", you killed the tension. The players know it is a deep one. No matter how awesome your descriptions will be, the mystery is gone. They know it's a deep one.

Instead you should take your time. Talk about the water being stirred up and about air bubbles rising to the surface. Let them react and ask you questions. Don't give straight answers, only give obscured half answers that will rise more questions.
The go ahead and maybe describe how that weird smell that has been in the air all the time is turning into an unbearable stench and how that thing underwater is now moving towards them at increasing speed.
Never tell them exactly what they are facing, or what is happening.

Give the players again the chance to react. Some of them might already have fled the scene.

This works of course better in games like Call of Cthulhu where characters have the habit of not surviving most of the critters out there, but that doesn't mean it doesn't work in other settings.

So take your time to build up tension! Keep the players guessing and give them plenty of time to react. Never fully describe what they see, keep it obscured by fog, rain, darkness, shadow. Don't let them know exactly what it is. The mystery will be more horrific and interesting than anything you will tell them.

On taking decisions for the players and railroading.

Railroading is most of the time used as a pejorative term. It means taking the decision away from a player or simply not allowing the player to act as he would like to, and by doing so destroying the main purpose of a roleplaying game.

There are situations though where taking minor decisions for the players can actually improve the game.
I use this from time to time to lead my players to a situations they would otherwise not have ended into.

Instead of asking the players, what will your characters be doing for the remainder of the evening? Where will they be eating? When will they be going to sleep? What will they be doing if they can't sleep?
You can go ahead and take some of the decisions for them.

"You had dinner in the Hotels restaurant and finished the evening in the smoking bar with some glasses of Brandy.
Around midnight, all of you are back up in your rooms. It's around 2am now and Max, you still weren't able to fall asleep. Your friends don't seem to have the same problem and you can hear them snoring through the thin walls of your room. You decide to have a walk downstairs and have a smoke in the hotel bar. Arriving on ground floor and turning to the main entrance hall, you see a trail of blood on the carpet leading to the door on the left end of the corridor."

I just took multiple unimportant decisions for the players and ended putting one of them into a nasty situation.
This is where you put the spotlight back on the player and have him now take the important decision.

You shouldn't do this if your players told you beforehand that they had specific plans for that night or that the decisions you are taking for a character goes against that players idea for his character. Else you end up railroading again.
Do it like Gandalf, "If you're referring to the incident with the dragon, I was barely involved.  All i did was giving your uncle a little nudge out of the door."

Pacing through scenes and transitions

Pacing is probably one of the most important aspects of storytelling.

You have to manage the time and effort you are spending on the different parts of the story.
You have to distinguish between "scenes" and "transitions".
A scene is where something interesting or important will be happening, where players will be finding clues, will meet one of the important npc's, will be dragged into a fight, will be advancing the plot or any other scene that is somehow actively involving the players. Remember they are not sitting around your table just to listen to your story.

A transition is the part between two scenes, like travelling from one place to the other, the days the characters spend resting, researching or celebrating etc. or any other trivial matter that will be going on during the story.
The transitions should be kept short. You can narrate it in one or two sentences, just to set the mood.
Example:
"After a short ride on the overcrowded subway and a long walk through the never ending rain you finally arrive at your Sire's villa."
This is always preferable than asking your players how they want to travel there, when they will leave, explaining them what they see on the way there and describing them the subway train in every possible detail. Save your energy for when the interesting parts come up.
It's during the scenes, where the interesting stuff is happening, that you should put a lot of effort in describing what they see, hear, smell and maybe even feel. Here you should take your time to set the mood for the scene and hand the acting over to the players again.

Some groups or players will actually enjoy to be able to roleplay their character during a downtime of a story, even if it doesn't relate to the plot. If that is the case, and they are asking for it, by all means let them play trough it. But you shouldn't be forcing this upon your players.

So it all comes down to highlighting the scenes of a story, putting weight on those parts, taking your time to describe, set the mood and play them trough, all while keeping the transitions short. Your story will advance  much faster and you will have less trouble keeping your players interested and attentive .
Think of it like a movie. If the movie is showing unrelated trivial stuff between the scenes, you will most probably be bored. And so will your players.

This has of course to be taken with a grain of salt. Nothing keeps you from describing a location beforehand, as an introduction for stuff that will be happening later on. Like handing out a map of the ship on which the characters will be spending the next month. Having them settle in and explore, and have stuff happen later on.
If on the other hand they just use the ship to travel from one place to the other, you can save yourself the trouble and skip to where you wanted to take them.

It's all about the players

When 5 people sit down around a table to spend an evening together rolling dice, they are expecting to be entertained and interact with the story. You, as the game master will bear most of the responsibility for everyone to have an enjoyable evening.
You have to keep in mind that the players want to have something meaningful to do, be able to develop their character and through the decisions they will be taking be able to influence the story .
Sitting there just listening to your story is not what the vast majority of the players will think of as fun.

So it will most of the time come down to focusing on the parts of the story where the players will have something meaningful to do. The parts in which they will be able to take decisions and roleplay their characters. The parts where the spotlight will be on them.
Your role will be to set the tone, the pace and draw the scenes for the players in which they will be acting.