I had a question come in recently on the use of the assistant first horn. I actually wrote a full article on this topic that was published a few years ago in The Horn Call, and it was for a time in my horn articles site. I pulled it offline after the publication of my high horn book, as I address the topic there.
In short it is not cheating to use an assistant! But speaking generally any passage that really has the first horn in the hot seat should be taken by the principal horn alone, with passages that resemble tutti/brass passages being played by the assistant alone so that the principal horn can stay fresh. If you are playing principal horn there is a wise usage that will extend your career and allow you to play at your peak more consistently.
Eldon Matlick of the University of Oklahoma has had posted online for some years [but see update] a very practical article on the topic of the use of the assistant horn, in which he notably includes as an example a description of exactly how he would use an assistant horn on Shostakovich 5. His opening passages serve well as an introduction to the topic.
I believe it was Anton Horner who first insisted on this position as a regular section player in the US. All succeeding Principals owe a lasting gratitude to him.
I attended a Master Class by Gail Williams on this very subject. She stated that the Assistant has probably the hardest job of all the section players. This is probably accurate. The assistant should be ready, and able, to perform all solo passages at a moment’s notice (Principal becomes indisposed for some reason). Also, having a strong assistant will allow the Principal to rest, reserving strength and/or lip to be ready for the solo or high danger passages which are notoriously difficult.
During these interim moments, the Assistant assumes the role of the section leader. However, the responsibility of the assistant does not stop there. Many times the assistant is also a utility player meaning that should any other member of the section become ill, or asks off a service, the assistant assumes that chair. Thus, the competent assistant should be aware/familiar will ALL four horn parts should any unforeseen catastrophe arise.
Assistants are sometimes maligned by egocentric Principals. It is too often that assistants are not used effectively, sitting for extended periods of time. There is an art to using an assistant, but more than that, a professional courtesy should be followed.
I am aware that, according to circumstances, all assistants are not created equal. However, intelligent Principals know how to effectively use their assistants without being heavy handed or self-serving. Remember how it would feel to sit for long periods of time and then be asked to play pianissimo entrances at the top of the staff.
There is much more in the full article, including his suggestions on Shostakovich 5. Thank you Dr. Matlick for posting this information in your site. And for those with access to back issues of The Horn Call my article “The Assistant First Horn in Orchestra and Band” was published in the February, 2004 issue on pages 33-34.
UPDATE 2011: The article by Dr. Matlick is no longer posted on his website, but the quotes above give the essence of the article.