Its a very subtle and difficult concept to really get hold of. The best way to see the issue is to look at a professional musician or group performing and you'll see the difference. It isn't about smiling or frowning or any specific thing. Its about owning the performance, presenting the music in a commanding way and vision is essential for that.
This is very true, and you've mentioned the key word 'professional' which of course means that owning and presenting the music etc. is essential. It is probably fair to say that because these people have chosen musical performance as a profession that this comes naturally to them.
Many brass bands have players of differing standards, ambitions and reasons for being there.
Bands that are made up of extreme standards and where the music is chosen to suit 'the average' is a recipe for uninterested / uninteresting looking players. Those at the higher end of ability will be bored, those at the lower end will be unable to play the part and therefore look anxious (or be miming!). These of course are not excuses but real reasons.
Of course we should make our players aware of the need to present as best possible, but beating ourselves up about something that isn't fully in our control uses energy better directed elsewhere.
Most audience members will usual latch onto 2-3 players when watching a performance regardless of whether there is a big choice. As long as a band has enough people that are interesting to watch (for good reasons), it doesn't matter a great deal if there are many that aren't.
The other thing to remember in an amateur brass is that most people are there to take their mind off their day job (I.e. relax) and enjoy making music. Too much ramming "perform, perform, perform" down their necks will not be healthy for band populations. For a professional musician, performance is critical. For an amateur it is important, but it won't be as important as the £10m deal they have to negotiate the next day.