For the most part, The Lost Weekend features the realistic production style that defined filmmaking in the 1940's. Within this basic framework, however, the movie takes the occasional flight of fancy that places us directly inside the mind of our alcoholic protagonist.
Sometimes this comes in the form of a perspective shot. There's one relatively early in the film when the camera dives straight into a shot of whiskey just as Don is about to drink it, submerging us in booze alongside of him.
Another notable example is the flashback at the opera house. Jonesing for the bottle of booze in his rain coat, Don hallucinates that the dancing opera performers are dancing trench coats, bottles of whiskey in their pockets. The actual visuals of this shot might be a tad cheesy by modern standards…but it's pretty effective in conveying Don's mindset.