How did armies convince men to stand in the front ranks (during late 18th and early 19th cen) during musket volleys? How could anyone just stand there? Why not lie down?

A mess of considerations towards fighting effectively. I first commend to your attention the appropriate section of the FAQ, specifically the headers 'Why did European armies use Linear Tactics?' and 'Mechanics of Linear Tactics'.

Adding onto that section, a few more posts:

u/StoryWonker has an overview of the relevant factors;
u/NotAWittyFucker also examines some of the relevant practicalities;
u/dandan_noodles looks at the dimensions of combat;
and on expands on the usage of column formations, examining the tactical situations at play and the question of motivation;
and on the degree of risk, also touching on motivation.