In fictional portrayals of ancient battles (Gladiator, etc.) the commanders are always personally fighting in the field. Obviously there would be situations in which the commanders are forced to fight alongside the troops, but what was the norm? When say, Roman emperors, would personally lead armies into battle it would seem too risky/costly for them to enter the fray, but perhaps it was more important to demonstrate their ability as warriors to the troops?
It depends on the culture, but in many cases it was the norm. In these two earlier thread, u/Iphikrates writes about how this was expected of Ancient Greek leaders. You can also read about the same phenomenon with mediaeval kings in answers here by u/Hergrim and here by u/MI13. The Romans were, as u/Iguana_on_a_stick points out in this answer, a bit different in that their leaders tended to stay back and usually come to the front ranks only when the situation was dire. To my knowledge reigning emperors tended to avoid partaking in battle for the first two centuries of Roman Imperial history