An example situation:
My friend is watching a horror movie. He is lost deep into the movie. Taking advantage of this situation I wear a mask of a ghost and suddenly appear in front of him just to scare him for the sake of fun.
Does the act I did ie. 'scaring a person for sake of fun' have a single word equivalent?
To my mind this sounds like spooking someone i.e. you put on the mask and spooked your friend.
Perhaps too specific for all the situations you might want this word for, but 'prank' is one word that leaps to my mind.
To play a practical joke on someone.
A practical joke is a mischievous trick played on a person, especially intended to make the victim appear foolish and experience embarrassment or discomfort.
The person doing the act is called a "practical joker."
I'd say "tease" is a basic word which covers most of the sense of what you're describing.
There are other words for it too, but they aren't family-friendly. :) You would probably hear them from your friend if you did something like this.
Spooked, but that doesn't always mean "for fun".
(For reasons I've never fully understood, people on almost all SE sites are very adverse to proposing negative answers, such as the one I'm going to propose here. Maybe there's a reason for that, but I figure I'll put it out there...)
I think the real answer to your question is "No, there is no one word to encompass the entire definition you are looking for."
I respect the intention of other answerers, to try to provide something approaching what you are after, but in the end, I think that misses the intent of the question. Any one of the suggested words, used on their own, will ultimately not accomplish what the questioner seeks, which is to use only one word and yet convey the complete meaning. "Spooking" does not contain any element that conveys "for fun". "Prank", "tease", and "practical joke" are not bound in any way to scaring someone, as they can be to simply surprise, embarrass, or cause laughter.
As much as I believe in, and love, the English language's ability to express any concept, that doesn't mean it has a built in word for everything. There are lexical gaps, which mean some things require a little explanation.
To convey what you want to convey, you will have to couch your phrasing in some amount of explanation:
I scared my friend just for fun.
Just for the heck of it I spooked my friend.
On a whim, I pulled a harmless, but scary, prank on my friend.
This isn't a limitation of English, or any language, it's the strength of it. Having a single word for every nuance of every little thing would be a maddening exercise of semantics. Modification through explanation is what lets us convey anything and everything without having to be simply walking definition databases.
Anyway, sorry to soap-box a little in the answer, but I'm a little confused by the hesitancy to not simply state when there is no word for something.
Also, as an aside, a question I'm inclined to ask out of curiosity is, "does any language have a word for that?"
Here are some of the words that could be used in a similar situation.
Prank, this is commonly used in schools and dorms.(if you don't believe me, go and live on campus)
If you wanted to go a little extreme, you could use wild prank
He looked about for some hint of a wild prank to play upon these strange, grotesque creatures that they might be again aware of his presence among them. Tarzan of the Apes by Burroughs, Edgar Rice
Jape, according to Oxford dictionary means a practical joke: the childish jape of depositing a stink bomb in her locker.
Freak where you freak (someone) out and you could be called a freak.
You ought to be forewarned, though: it looks like Star Wars in there. Monitors. Bells and whistles. So don't let it freak you out, okay? It just means they're taking real good care of him. from Wish You Were Here.
I would say that I bugged my friend. The word bugged comes from the word bugger which also means to make someone fear or to Poke someone.
