Attached Link: V4 Expressions (DAZ)

There are two (or maybe three) sides to this. Firstly, there's the basic technical requirement that the expression pose shouldn't alter any of the morphs that the character maker has used to create the facial characteristics. As you know, saving a so-called expression to the face library within Poser, if you take no further steps, will include channels for all the face morphs so some care is needed.

However, added to that is the unfortunate fact that some character creators will use morphs which are traditionally thought to be part of the expression (smile, blink etc.) as part of their face shape. Obviously even the most carefully crafted expression pose is going to interfere with those if it uses the same morph channels.

Second (or third), the morphs for any figure are created around the basic head shape. There's an art to making morphs that will sit well with a wide range of head shapes, but even so, there are going to be some characters where a preset expression isn't going to work as well as it does on the stock figure. Some tweaking is only to be expected and I believe some vendors even supply modified expression presets for this reason.

One of my favourite expression packs for V4 is Joelegecko's, at DAZ. They're mostly quite subtle and don't interfere with a character's face too much, but even so I generally tweak some aspect of them after application.