It's pretty common to see a King or second to the throne inspiring his troops from the front lines, before leading the soldiers into battle from the head of the pack. Was this common? If not, did it ever happen? How frequently would Kings actually join the fray?
hi! fyi, you'll find a few examples in this section of the FAQ
Yes, you can see it happening when England was threatened with the Norman and Viking invasions under Harold Godwinson, who died in the final battle he fought against the Norman invasion in 1066 at Hastings.
As recent as the Georgian period (1700s), did you have the last British monarch personally lead his troops into battle, which was under George II. http://www.bbc.co.uk/history/historic_figures/george_ii_king.shtml
In 1743, George led his troops into battle against the French at Dettingen, the last British king to fight in battle during the War of Austrian Succession.
Something that is portrayed in this painting.
Although later on around the Enlightenment, generally it appears that by the changing nature of war to be a more 'gentlemanly' affair where you have the officers holding back, ordering where troops go yonder, the risk of dying in battle decreased. The last English monarch to die in battle is, famously, Richard III, who died at the Battle of Bosworth Field.