If I had to make an hour and a half long movie out of a 16 page story, I'd approach the project with a system. First, time-wise how long does the story take? I'd create a timeline of events from start to end and realistically write down how long each of those events should take in a film format. From there it'd be about filling in any gaps. Given the brevity of the story, one can imagine some of the mundane or typically unimportant facets wouldn't be written. I'd try to fill in these gaps while adhering to the style of storytelling the author has. Then I'd stretch out what I could within the already established events