Harmonization
Harmony is one of the fundamental elements of music. Harmony provides a lot of the foundation of the song, as well as richness and much more. When creating your chart, following harmony is usually a very effective technique, as it is often times a detail that listeners ignore.
The highlighted synth melody begins as a solo, but when the chords in the chart come in, you will notice that the synth is no longer playing an individual note, but is playing two notes together, as a harmonic chord. As soon as the harmony leaves and goes back to being a single note, so does the chart.
There are several ways to chart a song to demonstrate the harmony. One method is through the use of miniholds, and another by direct layering. It might be difficult to chart the melody and harmony by layering because of the limitations on the number of columns available and in some cases, the density due to the number of voices and speed. In some cases, sacrifices will have to be made if the notecharter decides the harmonization is important. Either certain voices should be combined, cut out, or only certain kinds of sounds should be layered.
Notice that the first highlight (in yellow) is where the solo section of this song begins, and it begins with a single melodic line. At the green highlights, there is now a harmonic voicing. A small part of the harmonic voicing section is highlighted in red, and this is where the layering technique is quite interesting. Notice that even though there are distinctly two melodic lines that are playing 16th note rhythms, doubles are only placed on the 8th notes. The reason this is desired is because if every 16th note was a double, then the chart would be too dense and too difficult. The reason this works is because only a mild increase in density and chords is needed to feel the harmonic voicing, because you still have musically interesting motion between the doubles and 16ths due to pitch relevance.
What makes a technique that is effective in creating a clear transition between unison and harmony is whether that technique can be identified through connotation in a way that it is unlikely to be associated with something else. Some songs like to switch between a single melody and a melody with harmony in order to give the melody more richness and color, a more full sound. The same idea can be applied to the chart, for example, directly adding layering for the newly added harmony in order to contrast sparse and dense patterns, or directing attention away from the other instruments to create a contrast in the focus of the chart entirely.
Many pop songs with a verse/refrain structure will usually present opportunities for this sort of technique. Verses are usually sung as a single vocal track, and may occasionally have a few phrases of harmony. The refrain usually adds much more harmony, with many simultaneous vocal tracks, to make the melody feeling more full compared to the verse.
The vocals begin as a solo line, but during the fills and also after the drums pick up, you can hear harmonization in the vocals, which are given an extra layer of notes, as each of the attacks are doubles instead of single notes.
Determining when and how to incorporate the harmony into your chart will add a lot of richness to a chart.