"Do you ever play that piece that you used to practice a lot?"
My mother asked me that question one day when I was home on vacation after having played several years with the Philadelphia Orchestra. I played a few excerpts for her and she said very quickly, "That's the one." Of course, it was the chorale from Brahms' First Symphony.
What a glorious moment for the trombone section! And a treacherous one, for all three parts!
Lewis Van Haney and I used to compare our experiences with various pieces. I remember he said that Leonard Bernstein wanted the notes in the chorale to be quite separate. Whereas, my experience with Eugene Ormandy was that he preferred a very legato interpretation. The exact meaning of the dots under the slurs has always been confusing to the trombone players. It is an indication that is more common in string writing. Obviously, even great conductors vary on the desired interpretation. Perhaps this is a prime reason to study very carefully existing recordings of any work that you are considering. There WILL be variations. The player must find a style that is in the "window of acceptability" while, at the same time, be able and willing to adapt quickly if there is any hint of a suggestion that a change is desired.
One of the major problems of the alto part is the entrance on the first note after such a long period of inactivity. One suggestion would be to discover the manner in which one has the most success attacking a high A. I trust that it goes without saying that all such studies should have the use of air as the primary concern. This implies the INHALATION as well as (and perhaps more important than) the exhalation. One can explore various methods of attacks (with and without use of the tongue, always making sure that the sound is started with the air not the tongue), dynamic (forte and piano and accents), vibrato (slide and jaw), etc. In other words, develop the best approach for the player and then carry that success to the style of Brahms.
There is much concern about playing the chorale "cold," but, once one gets a good feel for the music, that fear is lessened. When practicing, play the music in your head as it leads up to the entrance. "Practice performing" the Brahms frequently, perhaps as often as the notes are played as an exercise.
One must not forget the tenor and bass parts. They are very important to the success of the performance. Van Haney used to call the second part, "The meat of the sandwich." A great tenor player can really make the chorale resonate. Seldom will a second player get a congratulatory comment from the conductor or a member of the audience, but an observation about how fine the "section" sounded is a direct notice of the importance of the second trombonist.
The bass line is very frequently used on major auditions. A fluid approach to the large intervals and valve changes is very important for the bass player. Practice of all the parts would help each of the performers. Also very important is the concept of playing the piece with a section! Balance, blend, and intonation can only be achieved with this approach.
Dynamics of each player are important to the blend and balance. I would suggest that, if the alto player is playing p, then the tenor player may need to play mp or even mf with the bass playing mp . Experimenting with the dynamics will indicate when the proper balance is achieved by the resonance of the chorale.
Of course, another consideration is whether to use an alto trombone [on for the first part]. And then, should the other trombonists (and perhaps the other brass players) use lighter instruments?
It is difficult to list all the possible approaches in this format. However, one idea is to do as my mother observed, "Practice a lot!"