Cfrgtkky

Screenplays are collaborative, whether you like it or not.

Actors will say the lines. Directors will alter the tone. The photographer will create their own vision. And the producers will hire other screenwriters to "fix" your screenplay because they have spent a great deal of money and will hire more experts than they need. The star will have some demands. Characters will be combined or discarded because of shooting schedule. And the year your screenplay goes into production a certain special effect will be trendy and an extra scene is added to showcase it. the studio has decided the film needs a soundtrack album of vintage hits from their back catalog. And the entire story has been transposed to Vancouver for budget reasons.

And then there is editing. And test marketing.

A screenplay is not a stand-alone document. screenplays aren't an "end in themselves" the way a novel is (comment: BruceWayne). A screenplay is really "spec" for more work in the industry, writing and fixing screenplays. As the writer you will not have much control over the final product.

As a novel author you might be asked to make changes in a rewrite, but a screenplay is not realized until long after it leaves your hands and has been reinterpreted in hundreds of ways by everyone involved.

You can pay to self-publish and you have a book. It will cost a lot of money to produce your screenplay.

Don't forget that nobody (except perhaps students on a film-making or screenplay-writing courses) ever reads screenplays, even those of classic famous movies. Write one by all means if you feel compelled to, but most likely the only person who will ever see it is you - unless someone else makes it into a movie.

As other answers have said, movies are a collaborative art form. Your screen play will just be the "basic idea" for what emerges at the end of the movie making process.

The same is true for writing stage plays, though maybe not quite to the same extent. But don't forget that even Shakespeare gets rewritten in modern productions. Scenes are cut, the order of other scenes is rearranged, somebody thinks it will be more "relevant for a modern audience" if Ophelia is black and Othello is white, or if Hamlet is female …

If you don't want else anyone to mess with your text, stick to writing books!

I'm gonna answer to the question in the title:

Ultimately it's up to you. They are different media.

Your friend must be right on some level.

I too have the habit to imagine my stories as movies (and I suppose it's not uncommon at all). Sometimes is painstalkingly difficult to portray with words what could be done with a good placed camera shot; writing is not a visual art after all. A good movie or even a good comic can impress the audience in a more immediate way; after all we relay a lot on our sight in our daily lives and as they say, a picture is worth a thousand words. Don't take me wrong - the written word still has its own merits and advantages, but this goes a little over the scope of the question.

The point about screenwriting is that, as you noted, it gets the story mostly out of your control. Let's say you write a really good screenplay and a company buys it. Let's say that you get activetly involved on the whole process of movie-making (rather than shoved aside). Even then, you won't be the only one responsible for the final product.
The screenwriter is not the director, in most cases; and even if you get to the point of directing your own film - unlikely for the first screenplay - there is a whole deal of people involved in the production and a whole lot of compromises to be made, from casting choices to business-related reasons, from the photography to the acting performance.

So it's true that becoming a skilled writer lets you keep the whole story into your own hands. No one else is involved in your book but you (apart from editors, of course; but I'd argue that a good editor can make your story better without altering your vision). If you get famous, if your book sells, it might one day become a movie; play your cards right and you could be one of the authors actively involved in the production. Of course, the same problems as above will apply, but maybe you'll be able to bargain a better deal if you have the higher ground of a strong sale record.

Then again, you should think about what your goal is.

What's more important? Telling a story, or making a movie? If it's the latter, try to get involved in movie making - it's the fastest route and it makes more sense. No point in taking an huge detour if that's what's your heart aches for. Your first screenplay won't get 100% right as you wanted it, and so for your second and third, but that's the same for everyone. And you'd be making movies.

But if you want to tell your story, as close as possible as you imagined it, and the visual representation would just be a nice collateral thing to have, you should write it down. Keep in mind that both ways will require sacrifices and compromises of some sort; it's up to you to choose.

Technology has potentially impacted this answer within the last 10 years. It's possible now to film a feature-quality film on a smartphone, which dramatically lowers the costs of making an indie film. Even so, making a movie is inevitably an expensive, time-consuming process involving multiple collaborators, and a host of uncontrollable variables.

You were given good general advice. That's not to say it couldn't have worked out to start with a screenplay. But your friend was correct that you have a LOT more command of the final product with a novel. The missing information here is what kind of writing do you most enjoy doing? And/or what are you good at writing? Screenwriting and novel writing are very different. If you enjoy writing novels, and are good at it, then your friend did you a huge favor. If your true love is writing for the screen, however, then it might not make as much sense to go the long way around. For instance, I've come to realize that, while writing books makes "more sense," in a general way, my own existing strengths (plot, dialogue, characters) and weaknesses (descriptive passages) match up better with screenplays.

It's inevitably a long, tough, journeyman's journey to become a successful writer, no matter which path you choose, so you may want to orient yourself towards your final goal, no matter what that is. But there's nothing that says you can't work on your novel and your screenplay simultaneously. Just remember they are different forms, and the same concept might be very different depending on the medium in which it's realized. The best adaptations have the flavor of the original, not the specifics.

I agree with all the other answers so far, but let me take a different perspective on the whole thing. If you wish, read this answer as a frame challenge.

You say "there's a movie in your head". What do you mean? Is it really a movie - does it fit into ~2 hours, do you see each frame and how it's shot, do you see the special effects and how they're done, do you have specific actors in mind?

Or is it just that you see every scene in vivid colour, full surround, like you're standing there?

If the first option is your case, I can see why writing a book rather than a screenplay would feel like you're not doing what you "want" to do, but "translating" your vision to a different medium, as it where. Maybe, if this burns in your bones, you'd want to accept the risks and disadvantages, and do the screenplay anyway. Get to know people in the industry, and follow your dream. (You might want to look at options like indie productions and animated films, as a way to mitigate the complexities of trying to move forwards with a screenplay.)

But if it's just that you see your story with the eyes and ears of your mind, it's not really a movie. It's just how you see the story. (And yes, the way we think of stories, and the way we dream, are both influenced by movies, as a matter of fact. For a while, when TV was black-and-white, people used to dream in black-and-white.) Since what you have in your mind is a story, go ahead and tell it. Tell it however you feel you can best tell it, however you would most enjoy telling it, however would let you tell it in a way that does most justice to the images in your mind. It could be a novel, it could be a graphic novel, it could be an epic poem - whatever medium is "yours", use that.

In another forum I've actually recommended to writers that they write the novel and the screenplay in parallel, even if you only intend to publish the novel. Writing the screen adaptation for a novel can expose weaknesses in the novel.

For instance, writers are constantly told "show, don't tell", because in the novel format it is depressingly easy (and common) to violate this admonition, and the failure to follow it generally results in weak prose. If you have told something to the readers instead of having shown it to them, when you write the screen play you will find it very difficult to depict the events in dialog and action. In such a case, you have to choose some other dialog and action that will advance the story (ie., raise the conflicts or develop the characters) in the way that you had intended in the same section of the novel. You can then turn that into prose and put it in place of the weak section of the novel.

Star Wars

The novelization of the first (released) film was actually released before the movie. Others have already discussed the theory behind why this worked. But the point that I wanted to make was that this is a proven path. Star Wars was one of the most popular movies of all time. And what's first on that particular list? Gone with the Wind -- another novel turned movie.

Were Star Wars and Gone with the Wind flukes? Another example would be Stephen King who has forty-two movies made from his work. Almost entirely book then movie. Part of this is of course that King paints intense visuals with words.

If you are already more comfortable writing in prose than the screenplay format, then that just strengthens this argument. Write the novelization of the movie in your head, as George Lucas had Alan Dean Foster do. Then use that to get whatever help you need writing the screenplay. The story can still be a movie. Just write the book of the movie.

TL;DR: Write the book first because that is a proven way of getting a movie made.