Okay, but how do you get tension? You get it with two characters in a struggle. If you've only got one character, how much tension can you have? The character is sitting there and thinking. What happens if she doesn't resolve whatever she's thinking about?
That's one way, and it's a popular and very useful way, but it's not the only way. One doesn't need a second character on-screen to have an active and vital scene.
Other, non-other-character, sources of tension:
+ Ticking clocks (in the story sense, though also in the real clock-sense, but then that would fall into the next item) (a character dying of cancer, frex)
+ Environmental cues
+ Puzzles that interest a reader
+ Voyeurism (not nec., but including, sexual)
+ A character doing something they shouldn't be doing.
+ The threat of personal harm, self-inflicted or from a non-sentient source + Language structured in such a way as to evoke tension.
+ Things lying beneath the surface; and, relatedly, the implication that beneath any surface is a layer we can't see (we like to think we know everything, but we know so little, and what we don't know frightens us; fright is a strong form of tension)
+ Reader knowledge where the character does not have knowledge
We agree, I think, that sitting and thinking rarely interests the reader beyond a paragraph. But there's a ton of other stuff that can happen while a character thinks through whatever's going on; the character doesn't have to be sitting in their bedroom, and the character categorically
does not need to be physically opposed by an antagonist character to create tension.
But I think if you look closely, you'll see that there's a time limit and a very real, very physical consequence -- he has to sort things out or the dragon will come back and fry the city or the psychopathic murderer from the future will kill his father so he'll never be born, so there's almost an implied antagonist in the room with him.
These are very workable, and very useful, techniques. They're not the be-all and end-all, though.
For most of us, if we try to write a scene with just one person (and, really, that's the problem, not the lack of dialogue; the lack of dialogue is the symptom of the problem), it's likely going to be of the "my life sucks, and I don't know what to do about it" variety. For the most part, keeping in mind that story = two characters in a struggle is going to improve any scene you write.
No offense, Jan, but your definition seems to be off.
"A story can be two characters in a struggle" is a valid statement, true.
Show me the two characters struggling through the island scene in Tom Hanks'
Cast Away, please? In
White fang?
A better definition might be "Story = the struggle of a character to overcomes frustration while actively seeking achievement of a goal." That's a useful definition, if not perfect.
An old line goes "'The King died, and then the Queen died'" is not a story; 'The King died, and then the Queen died of grief' is."
To wrap up: I think you're right in that your method will produce a scene, but I also think it's not the only way. I recommend that you look deeper into exactly what it is that you respond as a reader to in a scene--not just the tension itself, but the source of the tension.