If you want to test the effectiveness of a horn part, simply sing it all the way through.
This tip comes from my wife, the professional horn player. Too many composers have the idea that horns simply are the chucks in a “boom-chuck” – which they can do very easily. Or that horns are simply harmonic filler for greater ideas. Or even worse, that their job is to play little bits of inarticulate twiddles as counterpoint to other experimental twiddling going on in what is certainly a grand intellectual exercise (at least in the mind of the composer).
But in the mind of the horn player, their instrument’s color and expression is like the human voice – and they take great pains to emulate that character. When you compose against that grain, the results are seldom effective, and they’re very unlikely to be performed with any enthusiasm or conviction. The horn can whisper, hum, sing lightly, emote, shout, and even scream. The proportions with which you ask its players to perform these variations of force should be about the same as one might hear from an opera singer (or a Shakespearean actor).
What’s more, the natural range of the horn is quite similar to an operatic baritone singer: uncertain very low notes, a secure middle range, and a very strained upper register, becoming ever more difficult and loud up to the top notes. The horn can go a bit higher than a baritone, but its progressive timbre is very similar in some ways. It’s not just tunefulness or breathing which must be evaluated, but also the question of secureness of range. You try singing a leap of a 7th from the middle of the staff to a note above it, and see how in tune and clear that note is. Then you’ll understand why horn players find such passages treacherous. Also test for viability of articulation and phrasing. If your idea can’t be sung, it may also be something better suited for another instrument rather than the horn.
Note the following features in the excerpt below from the 1st horn part from my orchestral legend “Maui’s Fishhook.” The higher the music gets, the closer together the intervallic steps. Extremely high notes are carefully approached. The phrasing and articulation is all very natural to the way the horn speaks, and the dynamic proportions have a degree of variation. Finally, though there certainly are some very high notes, most of the part is within the natural range of the first horn, the notes inside the treble staff.