When you create a line of units these always have a small gap between them. This is annoying in case of phalanx units, since they are supposed to stand in a complete line. Some claim that this has an adverse effect on the value of phalanx units because enemies get in the gap and can that way flank the unit. In addition, it looks silly. But there's an easy way to fix it.
In the file descr_formation.txt, all formations are listed. If you change the variable inter_unit_spacing for line formation from 2.0 to 0, the gap disappears if you put units in line formation. While it works nicely for vanilla phalanx units several things should be noted:
- it will affect all units put in line formation. That shouldn't be a problem since the missing gap will hardly hinder them. If you still want the option, you can replace a less important formation with the original line formation and use that one for non-phalanx units.
- units that have an officer like most elite units will still have an empty space behind the officer.
- only your units will be affected, not the AI, which may create an advantage. But since the AI can't handle phalanx anyway, it shouldn't make much difference. It might be possible to do this for the AI as well, by removing the unit spacing in all relevant formation blocks in the AI formations file. But I haven't tested that yet.

I haven't tested yet whether closing the gap does indeed make phalanx units more effective, but alone the looks are worth it