Rubato means managing the rhythm flexibility, therefore varying the duration of the written notes.
Used as a concept of flexibility and elasticity of tempo it naturally becomes an instrument of expression.
Being the consequence of a feeling, it expresses a state of mind which becomes more definite in its expression.
For this reason rubato cannot always be done, and it cannot always be done the same way.
No pianist begins Chopin's Nocturne Op. 9 N. 1 in a metronomic way, without rubato.
Chopin does not write rubato, but he writes other things - the way he manages the various lines, the attention to the intervals between the notes of the same melody - which suggest that rubato has to be done.
Rubato must not be accidental but rather enhance the differences of interval and harmonic tensions.
The first six notes that open the Nocturne in such a bare way, without an accompaniment of the left hand, feature different intervals: an ascending tone, a semitone (more dissonant than the tone and therefore more "suffering") and then a descending diminished fourth. It would be plausible to play it without rubato if Chopin had written it using more consonant or more regular intervals.
But Chopin’s music is so special because intervals and harmonies are often used in an unexpected way, as a means to express complex emotional situations.
Already the first high D♭, the highest note of this first phrase, represents the expression of something interior. You can slightly linger on the note C that precedes it and also remove some dynamics from the D♭ itself, so that it sounds more internal, nostalgic, and less affirmative.
Then there is the diminished fourth - D♭/ A ♮ - which cannot be performed uniformly, otherwise we will lose the special, unique feeling of this interval.
The phrase continues with four consecutive Fs and then it rests on a G♭ which Chopin indicates with a small accent in the score. In a melody the dynamic indications (an accent, a sforzato, etc.) become at the same time also agogic because they concern the expression of feelings. This is not an accent to be played only louder, and which would become unnecessarily aggressive, but on the contrary it has to sound as an internalized accent. In practice this means lingering on the G♭, making it last a little longer, and then fading later.
In fact these are all rubato which demonstrate how rubato is not something purely instinctive, left to the talent or inspiration of the moment. It must be natural and spontaneous, but it must also be part of the complex language of classical music.
It is up to the interpreter to deduce this from the author's indications and from the score’s intervallic and harmonic tensions.
Photo: Chopin Nocturne Op. 9 N. 1 First edition, Fr. Kistner, 1833.