You can freely disengage and use their command abilities. Importantly, though you can expect flashy animations as the heroes battle, they aren't locked into that encounter. There isn't a Three Kingdoms duelling mechanic here either, but if two great heroes happen to meet on the field, warriors will back off and let the legends battle it out. Combat encounters build 'rage' for Troy's heroes, which you then use to unleash their command abilities to inspire nearby units. They can't fend off an entire unit as heroes could in Three Kingdoms, but you want to throw Odysseus into a scrap with his warriors. Vasilev describes heroes as being great close support units. Sometimes infantry scraps can become one giant static mob of warring units in Total War, but Troy's famous heroes might be able to resolve these situations a little bit faster. But there's a balance to strike with the game's pacing. Fights won't be as immediate as Total War: Warhammer-no-one's throwing fireballs around in this game. The developers are tweaking this carefully. This should hopefully lead to more decisive infantry encounters.
Flanking debuffs will matter more in Troy, flank attacks will damage morale and reduce the targeted unit's defence stats. They are also great at flanking units engaged in hand-to-hand combat. Light units tend not to get exhausted, they move easily over difficult terrain, and they can hide in long grass to ambush units. "In Troy a light unit might be twice as fast, maybe even more than twice as fast," says Vasilev, "so you will really feel that difference when it comes to their speed." Light units give you speed and flanking capability.
There's mud, too, which is particularly difficult for heavily armoured warriors to negotiate. Troy's maps contain more blocking features such as cliffs and valleys, which of course affects angles of attack. You also need to be mindful of terrain when arranging your infantry. These units will be slow, and vulnerable to exhaustion, which is even more important in Troy.