To be a Principal Flutist, one must have style, spontaneity, variations of tone colors, a sense of ensemble with other principal winds, and all the standard musicianship qualities, such as technique, rhythmic sense, solid intonation, etc. To be a Second flutist, one must have all these qualities in addition to the foremost quality of "flexibility". At any moment, a second flutist can find himself/herself playing a solo with the string section, with the second oboe, principal or second clarinet, first oboe or first horn, or might be almost entirely alone. Another moment the second flutist could be playing a tutti passage, and another, the hardest of all, in unisons, octaves, thirds, fifths, sixths, with the principal flutist, sometimes as chords, sometimes as melodic lines. Sometimes the second flutist alternates solo passages with the first. All this means incredible flexibility.
Of course, the first ingredient to creating success in attaining this flexibility is having an attitude of team play. The second flutist must follow the lead, imitate the style, tone color, phrasing, and rhythmic sense of the first. The attitude is one of pride in ensemble and pride of the ability of matching another person's way. Sometimes this situation calls for sacrificing one's preference of tone colors, phrasing, and often, beliefs about where to place the pitch. Sometimes it requires a willingness to play out of tune with the orchestra in order to accommodate the first player. There must be a willingness to play softer or louder than desired, shorter or longer, faster or slower, more rubato or less, more vibrato or less, more soloistic or less, etc. This awareness of what is going on is a spontaneous reaction to the surroundings. Not two performances are exactly alike in real life, and the second player can be expected to accommodate instantaneous changes. It, therefore, requires a receptive attitude and one that is alert and instantly ready to respond. The second player often has to perform with "rubber lips" to adjust to pitch and color, and be willing to sacrifice one's own purity of sound to accomplish a blend. Instinct is the key factor.
Judgment is another factor. When is a dynamic level supportive, weak or too loud? The second player sets the foundation for the first. Without confidence and fullness in the supporting note, the principal cannot float on top. Obviously, an aggressive, harsh or overpowering dynamic makes the first force, and a weak wishy-washy approach makes the first stick out. Likewise, in rubato or long phrase lines, a line that slightly lags behind the first feels like prison chains, but pushing too far forward can cause a lack of musical poise and can even cause the first to rush or the ensemble to collapse.
The art of playing second is a mighty one. Not many know the joys of the feelings of personal success in this art. Most outward credit goes to the principal player. When the second really is artistic at the task, the principal is free to play at his/her best. It can be said that "behind every successful principal's performance, is a great supportive second chair player."