It is pretty easy to guess why most people consider video games to be very close to movies as a medium: both use images to convey their stories. Yes, the "closeness" does not go farther than that.
Or, should I rather say, should not go farther than that. Indeed, I do believe that we should consider video games to be, as a storytelling medium (this analysis does not apply to the very large number of games that merely don't need a story), treated as much closer to books than movies. To prove my point, I will analyze the main characteristics of each medium regarding specific points.
Length: How long does it take to go through the story?
Movies are ultimately a short medium. Most are 1h30 long, some can be 2, 3, rarely 4 hours long. Basically, in one afternoon or evening, you have watched the whole movie.
On the other hand, books take a lot longer. Now, it depends on how long the book is, and on how fast a reader you are, but we can all agree that most of the times, it takes around 1 to 2 weeks to read a book. Of course you can stumble upon a book that is so good that you'll read in a couple of hours, but this is not really the norm.
It works exactly the same way for video games. I usually finish a game in 1 to 2 weeks, but can stumble upon a game which is so great that I'll finish it in one day, even though it is pretty rare.
In movies, you don't have a lot of time. You need to go fast, cut away everything you don't absolutely need. The introduction rarely lasts for more than 20 minutes, and you can’t have that many twists and turns in the plot. That is why most movies only have pretty simple, straightforward stories, otherwise they may quickly lose their audience under the amount of information you want to transmit to him.
But when it comes to either books or movies, you have all the time in the world. Some books can have up to two, three thousand pages, while some games can last up to sixty or more hours. It is not uncommon for these mediums to have 5 to 10 main characters, all with their own developped personnalities and backstories - something movies can't sustain.
Everything can happen a lot slower, and the introduction can last longer. Obviously, you don't want a one hour long introductory cutscene, but you can afford to have 1, maybe 2 "prologue" levels before getting into the real action. Look, for example, at Fire Emblem: Path of Radiance: the actual plot does not start before the fourth mission, which is already, for most players, 1 or 2 hours into the game.
The length of this medium also have a strong impact on its rythm, something I'll cover right now.
Rythm: What is the rythm of the story?
Since a movie is meant to be watched in a single stroke, with no interruption, most movies will follow the same rythm: strong introductory scene to get the spectator in (which is often loosely related to the main plot or will only make real sense halfway or more through the movie), then slow introduction to the characters and situation, exponential build up during most of the movie, climax and conclusion.
On the other hand, both books and movies are meant to be enjoyed over a long time, with long interruptions between every time you read / play. This is why you need a different rythm for the story.
You still have one big, main plot that slowly builds up until reaching the climax, but as it is stretched over a long time and meant to be enjoyed by small servings, you also need another, smaller and more complex organization for the story. Both books and video games are divided into chapters. Of course, the division is not always that obvious in games, but think about it: every game has either levels, dungeons, missions, quests... And each of these chapters actually contain a subplot, with a small climax and often a cliffhanger to make sure the reader / player wants to come back for the next chapter.
This forces you to cut your story in small, bite-sized chunks, but also allows you to develop one subplot for each of them, thus enriching your main plot. You can even dedicate one of these chapters to a particular character, allowing you to really develop every one of them much more than you could in a movie.
Communication: How is information relayed?
In movies, everything relies on what is shown and what is told. You can use a little bit of text (like the name of a place or some information shown on a panel) but rarely anything more. Of course some exceptions exist, such as the iconic Star Wars introduction, but they are what they are: exceptions.
Dialogs need to be short, as the watcher cannot follow a conversation that is too long. You can have very few monologs, and simpler is always better. The movie also relies a lot on the performance of the actors, and how good they are to relay emotions.
Books, on the other hand, rely on text. Everything is written down, every line of dialog and every description. When two characters are talking, it is up to the reader to picture their attitudes, emotions etc.
When it comes to video games, thanks to (or because of? this is a question I'll raise in my next blog post) technology, voice acting has become a norm in most games. But a lot of data is still transmitter through text, and not always unimportant data (quests, lore details etc). But acting is still very limited in games, and only a handful of them have character models good enough to convey actual emotions.
Imagination: What room is left for fantasy?
Movies, of all mediums, are those who leave the less room for imagination. Everything is given to the spectator, every single detail of every single place / action.
On the other hand, books rarely describe everything. A lot is left to the interpretation of the reader, who usually likes personnalizing things a bit.
Same goes for games. A lot of games allow you to customize the main character, but things go even farther than that. It's up to you to decide how things actually happen. The game or the book will only tell you: he fought and defeated 10 foes at the same time, and it's up to the reader to imagine the battle and to the player to fight the battle.
Movies are also very limited by their race toward realism. You can't have settings that look too fantastical, or the CGI will soon appear too obvious. You can't have a main character that looks nothing like a human, or you'll have trouble finding an actor for it. Of course you can still do an animated movie, but this means you'll aim for a very specific public (children).
In books AND in games, you can imagine anything you want, and create it. In books, it's up to the reader to imagine it, and in games, since everything is already virtual, you have no risk of having your settings look too unreal.
Sequels: What is the focus?
Movies are extremely limited when it comes to sequels. You often need to keep the same actors, and the plot has to be pretty close to that of the first movie. And you have your hands bound by the fact that actors do get older, so if you sequel is done 15 years after the first movie, you'd better find a story that takes place 15 years after the first one (of course you can still use make up to try and make him look younger or older, but only to a certain degree). And the spectators often create strong links between a movie and his main character, which means the actor playing this role.
Books and games have much less limits when it comes to making sequels, and don't require to stick with the same character (and when they do, are not bound to make the sequels all happen in near chronological order). I think this is why both these medias make a heavy use of sequels (yeah, there are many books that are part of cycles or series). And you don't have to stick with one single main character: as these stories do not rely that much on the actor's face and performance, the reader / player's focus can be shifted to any other element you want.
So, these where my two cents in a debate I hope will pick up, as it can actually help shape storytelling methods and tools in this still very new media.